CMOS amplifier circuit and CCD delay line with CMOS amplifier
BACKGROUND OF THE INVENTION 
     The present invention relates to a CMOS amplifier circuit formed by a CMOS semiconductor process, and to an output circuit of a charge-coupled device (CCD) delay line which includes the CMOS amplifier circuit. 
     Recently, integrated circuit devices (ICs and LSICs) formed by a CMOS semiconductor process have been extensively employed because they have excellent characteristics. Namely, CMOS circuits have a wide operating voltage range and are low in power consumption. CMOS amplifier circuits formed by a CMOS semiconductor process are applied to internal signal amplifier circuits, output power amplifier circuits, and so forth. 
     A CMOS amplifier circuit AMP shown in FIG. 7 is known in the art. Specifically, the CMOS amplifier circuit is an active load type source-grounded inverting amplifier circuit. A P-channel MOS transistor Q 1  serving as an active load has its gate and source electrodes connected together. An N-channel MOS transistor Q 2  is connected between a high voltage source V CC  and a low voltage source V EE  (V CC  &gt;V EE ). A signal V G  is applied to the gate electrode of the MOS transistor Q 2  and subjected to inversion and amplification to provide an output signal V 0  at the common connecting point of the transistors Q 1  and Q 2 . 
     It is assumed that the mutual conductance of the MOS transistor Q 1  is represented by g mL , the resistance between the drain and the source thereof is represented by r dsL , the mutual conductance of the MOS transistor Q 2  is represented by g mD , and the resistance between the drain and the source thereof is represented by r dsD . Hence, the output impedance Z 0  of the amplifier circuit is as follows: ##EQU1## The voltage amplification factor A V  is as follows: 
     
         A.sub.V =V.sub.0 /V.sub.G =-g.sub.mD ×Z.sub.0        ( 2) 
    
     In the amplifier circuit, the parallel resistance of r dsL  and r dsD  has the following relationship: 
     
         (r.sub.dsL ×r.sub.dsD)/(r.sub.dsL +r.sub.dsD)&gt;1/g.sub.mL( 3) 
    
     Therefore, in above-described equation (2), the voltage amplification factor A V , can be rewritten as follows: 
     
         A.sub.V ≈-g.sub.mD /g.sub.mL                       ( 4) 
    
     The above-described CMOS amplifier circuit is advantageous over a source-grounded inverting amplifier circuit in that it can be lower in impedance and it can be miniaturized. 
     The signal V G  applied to the gate electrode of the MOS transistor Q 2  is generally supplied from a differential amplifier as shown in FIG. 7. The differential amplifier includes N-channel MOS transistors Q 3  and Q 4  forming a differential pair a constant current circuit I 0 , and P-channel MOS transistors Q 5  and Q 6  forming an active load. An input signal V I  from- various circuits (not shown) is applied to the gate electrode of the MOS- transistor Q 3 . 
     The amplifier circuit can also be employed as a voltage follower circuit which has improved linearity with the output signal V 0  fed back to the gate electrode of the MOS transistor Q 4  on the non-inverting input side of the differential amplifier. 
     However, the above-described conventional amplifier circuit suffers from certain problems and disadvantages which are discussed below. 
     First, as shown in FIG. 8, the threshold voltage V th  of the MOS transistor Q 1  connected to the high voltage source V CC  causes compression of the output DC dynamic range on the side of V CC . 
     Second, in the case where the CMOS amplifier circuit is used as a buffer amplifier where a plurality of circuits are connected to it in subsequent stages, it is desirable that the output impedance Z 0  be low. However, since the voltage of the output signal V 0  is increased with the decreasing voltage of the input signal V G  of the MOS transistor Q 2 , the bias voltage V GS  between the gate and the source of the MOS transistor Q 1  is decreased, and the mutual conductance g mL  is also decreased. Therefore, as is apparent from the above equations (2) and (4), as the mutual conductance g mL  decreases, the output impedance Z 0  increases. As the output impedance Z 0  increases in this manner, the high-frequency cut-off frequency f H  is decreased. As a result, the circuit may become unstable, resulting in undesirable oscillation. 
     Third, the capacitance between the gate and the drain of the MOS transistor Q 1  forms a capacitive load on the MOS transistor Q 2 . Hence, the high-frequency cut-off frequency f H  is lowered by the capacitive load, thus making it difficult to increase the bandwidth of the circuit. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing an object of the invention is to provide an active load type source-grounded inverting amplifier circuit which is superior to conventional active load type source-grounded inversion amplifier circuits in both dynamic range and bandwidth. 
     Another object of the invention is to provide a CCD delay line with excellent characteristics and which has a CCD amplifier circuit as its output circuit. When employing a charge-coupled device (CCD) as a video signal delay line, it is essential to employ an output circuit which has a wide frequency band and a low output impedance. 
     The foregoing objects and other objects of the invention have been achieved by the provision of a CCD amplifier circuit including an active load type inverting amplifier circuit which includes a driving MOS transistor, an active load MOS transistor connected to the driving MOS transistor, and a control circuit for controlling the voltage at the gate electrode of the active load MOS transistor with a control signal so as to provide low output impedance, the control signal being substantially inversely proportional to the drain-source voltage of the active load MOS transistor and is level-shifted by a predetermined voltage. 
     In an example of the CCD amplifier circuit, the gate electrode of an active load MOS transistor is driven by the output of a drain-grounded buffer circuit having a gate electrode to which the drain-source voltage of the active load MOS- transistor is applied, and having a source electrode which is connected to a constant current source. 
     In addition, in a CCD delay line in which a floating diffusion region of predetermined impurities is formed at the end of a charge-coupled device with a gate section having a predetermined fixed gate voltage, and a switched capacitor integrator for detecting the injection charge of the floating diffusion region is provided so as to detect signal charges transferred to the floating diffusion region from the charge-coupled device. The above-described CCD amplifier circuit is employed in an output circuit of the switched capacitor integrator. 
     In the CCD amplifier circuit thus constructed, even when the output signal increases in voltage amplitude, neither the mutual conductance of the active load MOS transistor nor the pinch-off voltage is decreased because a forward bias voltage is applied between the gate and source electrodes of the active load MOS transistor at all times. 
     Furthermore, because the gate capacitance of the active load MOS transistor is charged by a control signal of a low output impedance control circuit, it does not become a capacitive load on the driving MOS transistor. Hence, the amplitude of the output signal is not collapsed by the effect of a pinch-off voltage, that is, the dynamic range is increased. In addition, as is apparent from the above expressions (1) and (4), the output impedance does not increase, while the high-frequency cut-off frequency is increased. Thus, the resultant CCD amplifier circuit has a wide bandwidth. 
     A CCD delay line capable of processing video signals having a wide bandwidth can be realized by applying the CCD amplifier to an output circuit thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following description in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram showing an example of a CCD amplifier circuit according to the invention; 
     FIG. 2 is a waveform diagram illustrating features of the CCD amplifier circuit according to the invention; 
     FIG. 3 is a graphical representation of operating characteristics of the CCD amplifier circuit according to the invention; 
     FIG. 4A is a schematic diagram of an example of a CCD delay line according to the invention; 
     FIGS. 4B and 4C are explanatory diagrams for the operation of the example of a CCD delay line illustrated in FIG. 4A; 
     FIG. 5 is a circuit diagram illustrating the output circuit of the CCD delay line of FIG. 4A in more detail; 
     FIG. 6 is a timing chart indicating control signals for controlling the output circuit of the CCD delay line; 
     FIG. 7 is a circuit diagram showing a conventional CCD amplifier circuit; and 
     FIG. 8 is a waveform diagram illustrating problems accompanying conventional CCD amplifier circuits. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An example of a CCD amplifier circuit, which constitutes an embodiment of the invention, will be described with reference to the accompanying drawings. 
     First, the arrangement of the CCD amplifier circuit will be described with reference to FIG. 1. 
     In FIG. 1, a circuit AMP is an essential part of the CCD amplifier circuit. The circuit AMP includes an N-channel MOS transistor Q 7 , which has a source electrode connected to a low voltage source terminal V EE  and a gate electrode to which a signal V G  to be amplified is applied, and a P-channel MOS transistor Q 8 , which has a drain electrode connected to a high voltage drain terminal V CC  and a drain electrode connected to a drain electrode of the MOS transistor Q 7 . An output signal V 0  is provided at the connecting point P of the two MOS transistors Q 7  and Q 8 . As an example, the low voltage source terminal V EE  can be ground and the high voltage source terminal V CC  can be 5 volts. 
     The circuit AMP further includes an N-channel MOS transistor Q 9  having a gate electrode connected to the connecting point P, a drain electrode connected to the high voltage source terminal V CC , and a source electrode connected to a gate electrode of the MOS transistor Q 8  and connected to the low voltage source terminal V EE  through a constant current source I 1 . 
     The circuit AMP thus constructed is coupled to a differential amplifier including, for instance, CMOS transistors and a constant current source, so as to be employed as a power amplifying buffer amplifier. The differential amplifier includes N-channel MOS transistors Q 10  and Q 11  forming a differential pair, a constant current source I 2 , and P-channel MOS transistors Q 12  and Q 13  serving as an active load for the differential pair. A gate electrode of the MOS transistor Q 11  is connected to the connecting point P. A drain electrode of the MOS transistor Q 10  is connected to the gate electrode of the MOS transistor Q 7 . An input signal V I  from an external circuit is applied to a gate electrode of the MOS transistor Q 10 . 
     The operation of the CCD amplifier circuit thus constructed is described below. 
     The MOS transistor Q 9  and the constant current circuit I 1  form a level shifting circuit which subjects the output signal provided at the connecting point P to power amplification and supplies the resultant output signal V X  to the gate electrode of the MOS transistor Q 8 . In the MOS transistor Q 8 , unlike its counterpart in the conventional amplifier circuit, the gate electrode and the source electrode are not connected to each other, and instead the bias voltage between the gate electrode and the source electrode is controlled by the output signal V X . 
     In the case where the amplitude of the input signal V G  goes towards the voltage V CC , the output signal V 0  is inverted and amplified, as indicated in period τ 1  shown in FIG. 2. Further, the output signal V X  is also inverted and amplified. Thus, the MOS transistor Q 8  is maintained forward-biased, and the gate electrode voltage is decreased. 
     On the other hand, in the case where the amplitude of the input signal V G  goes towards the voltage V EE , the output signal V 0  is inverted and amplified, as indicated in period τ 2  shown in FIG. 2, and the output signal V X  also shows a waveform inverted and amplified. Thus, the gate electrode voltage of the MOS transistor Q 8  is increased. In this case, the output signal V X  is level-shifted by an amount equal to the gate-source voltage V GS  of the MOS transistor Q 9 , and accordingly the gate electrode voltage of the MOS transistor Q 8  is lower than the voltage of the output signal V 0  by as much as the level-shifting voltage V GS . Thus, the MOS transistor Q 8  is forward-biased at all times and never pinched off. 
     Accordingly, as indicated in FIG. 2, the waveform will not collapse, that is, the dynamic range is increased as compared to the dynamic ranges of conventional circuits. In addition, the mutual conductance g mL  of the MOS transistor Q 8  is not decreased because the MOS transistor Q 8  is maintained forward-biased at all times, as described above. Therefore, as is seen from the above equations (1), (2) and (4), the output impedance Z 0  can be maintained low and the amplification factor A V  can be increased. 
     The gate electrode capacitors C DG  and C SG  of the MOS transistor Q 8  are charged by the low impedance signal V X . Therefore, the capacitive load of the driving MOS transistor Q 7  is reduced, and the high-frequency cut-off frequency f H  is increased. Hence, the amplifier circuit is free from oscillations, stable in operation, and it has a wide bandwidth. 
     The frequency versus gain characteristic of the CCD amplifier circuit has a wide bandwidth characteristic and a high-frequency cut-off frequency f 2 , as indicated by a characteristic curve A V1  in FIG. 3. The frequency versus gain characteristic of the differential amplifier shows a wide bandwidth characteristic (with a high-frequency cut-off frequency f 1 ) as indicated by a characteristic curve A V2  in FIG. 3. In the case where the output voltage V 0  is fed back to the differential amplifier in order to improve linearity, phase compensation can be achieved by suitably adjusting the gate-source capacitance C P  of the MOS transistor Q 7  in such a manner as to shift the cut-off frequency f 1  from the cut-off frequency f 2 . 
     Now, an example of a CCD delay line using a charge-coupled device will be described. The CCD delay line according to the present invention is an improvement of the CCD delay line which the present inventor has described in Japanese Patent Application No. 103100/1990. 
     First, the structure of the delay line will be described with reference to FIG. 4. 
     As shown in FIG. 4, an N -   impurity ion implantation layer 2 is formed in the surface of a P-type semiconductor substrate 1, and charge transferring gate electrodes are formed thereon through a gate oxide film. Thus, a buried channel CCD (BCCD), which is the body of the delay line, is formed. In FIG. 4, gate electrodes 3, 4, 5, 6, 7 and 8 are shown as part of the output section of the BCCD, and drive signals φ 1 , φ 2  and φ 2A  are applied to the gate electrodes to transfer signal charges. The drive signals φ 1  and φ 2  are transfer clock signals in a two-phase drive system, and the drive signal φ 2A  is a clock signal which becomes positive or negative in synchronization with the signal φ 2 . 
     A gate electrode 9 is formed adjacent the gate electrode 8. A predetermined DC voltage OG is applied to the gate electrode 9. In this example, the predetermined DC voltage OG is zero (0) volts. In addition, an N +  impurity layer 10 is buried in the surface of the P-type semiconductor substrate 1 adjacent the gate-electrode 9. The impurity layer 10 is connected to an output circuit. 
     The output circuit of the CCD delay line includes differential amplifiers 11 and 12. The differential amplifier 11 has an inverting input terminal connected to the N +   impurity layer 10, and a non-inverting input terminal receiving a predetermined bias voltage V B  (e.g., 3 volts). A parallel circuit of a capacitive element 13 and an analog switch 14 is connected between the inverting input terminal and the output terminal of the differential amplifier 11. Thus, the differential amplifier 11 forms a switched-capacitance integrator. 
     In the differential amplifier 12, the non-inverting input terminal is connected to the output terminal of the differential amplifier 11, and the inverting input terminal is connected to an output terminal of the output circuit. Thus, the differential amplifier 12 serves as a buffer amplifier. 
     The output terminal of the differential amplifier 11 is connected through an analog switch 16 to the non-inverting input terminal of the differential amplifier 12. The non-inverting input to the differential amplifier 12 is grounded through a capacitive element 17. The analog switch 16 and the capacitive element 17 form a sample and hold circuit. 
     The output circuit is shown in more detail in FIG. 5. As shown in FIG. 5, the differential amplifier 11, which forms the switched capacitor integrator, includes MOS transistors Q 14  through Q 22 . The analog switch 16 includes MOS transistors Q 23  and Q 24 , and an inverter 18. The buffer amplifier 12 includes MOS transistors Q 25  through Q 33 . 
     The differential amplifier 11, which forms the switched-capacitance integrator, is constructed as follows. In the differential amplifier 11, the N-channel MOS transistors Q 14  and Q 15  form a differential pair which is connected to the N-channel MOS transistor Q 16  and to the P-channel MOS transistors Q 17  and Q 18  serving as active loads. The N-channel MOS transistor Q 16  serves as a constant current source when controlled by a fixed bias voltage V BIAS . The drain electrode of the MOS transistor Q 15  is connected to a CCD amplifier circuit (corresponding to the amplifier circuit AMP in FIG. 1) including the MOS transistors Q 19  through Q 22 . 
     In the CCD amplifier circuit within the differential amplifier 11, the transistor Q 19  is a driving N-channel MOS transistor, the transistor Q 20  is a P-channel MOS transistor serving as an active load, and the transistors Q 21  and Q 22  are N-channel MOS transistors forming a level-shifting circuit. The N-channel MOS transistor Q 22  forms a constant current source when controlled by a fixed bias voltage V BIAS  (e.g., I 1  in FIG. 1). A capacitive element C P1  is connected between the gate electrode and the source electrode of the driving MOS transistor Q 19  for the purpose of phase compensation. 
     A parallel circuit of a capacitive element 13 and a P-channel MOS transistor 14 is connected between the gate electrode of the MOS transistor Q 14  and the drain electrode of the driving MOS transistor Q 19 . The fixed bias voltage V B  is applied to the gate electrode of the MOS transistor Q 15 . The gate electrode of the MOS transistor Q 14  is connected to the floating diffusion region 10. 
     The analog switch 16 forming the sample-and-hold circuit includes a P-channel MOS transistor Q 23  which is rendered conductive when the sample-and-hold control signal SH is at the &#34;L&#34; level, a P-channel MOS transistor Q 24  serving as a gate electrode capacitor and connected to the source electrode of the MOS transistor Q 23 , and an inverter 18 for inverting the sample-and-hold control signal SH and supplying the inverted signal to the gate electrode of the MOS transistor Q 24 . A capacitive element 17 is connected to the source and drain electrodes of the MOS transistor Q 24 . Thus, the sample-and-hold circuit is formed. 
     The differential amplifier 12, which forms the buffer amplifier, is constructed as follows. The N-channel MOS transistors Q 25  and Q 26  form a differential pair. The differential pair is connected to the N-channel MOS transistor Q 27 , which serves as a constant current source when controlled by the fixed bias voltage V BIAS , to the P-channel MOS transistors Q 28  and Q 29  serving as active loads. A drain electrode of the MOS transistor Q 25  is connected to the CCD amplifier circuit (corresponding to the amplifier AMP in FIG. 1). 
     In the CCD amplifier circuit within the differential amplifier 12, the transistor Q 30  is a driving N-channel MOS transistor, the transistor Q 31  is a P-channel MOS transistor serving as an active load, and the transistors Q 32  and Q 33  are N-channel MOS transistors forming a level-shifting circuit. The MOS transistor Q 33  forms the constant current source when controlled by the fixed bias voltage V BIAS  (e.g., I 1  in FIG. 1). A capacitive element C P2  is connected between a gate electrode and a source electrode of the driving MOS transistor Q 30  for the purpose of phase compensation. 
     The output signal of the sample-and-hold circuit is applied to a gate electrode of the MOS transistor Q 25 , and the gate electrode of the MOS transistor Q 26  is connected directly to the output terminal of the differential amplifier 12. Thus, the differential amplifier serves as a negative feedback buffer amplifier. 
     The operation of the CCD delay line thus constructed will be described with reference FIGS. 4B, 4C and 6. FIGS. 4B and 4C show potential profiles with respect to the gate electrodes. 
     In order to read signal charges transferred having a predetermined period in synchronization with the transfer clock signals φ 1  and φ 2 , at the beginning of each period τ unnecessary charge is removed from the capacitive element 13 connected to the differential amplifier 11, and the potential of the floating diffusion region 10 is initialized. For instance, at the time instant T1 in a period τ, the reset signal RST is raised to the &#34;H&#34; level to render the analog switch 14 conductive thereby to set the voltages at the inverting input terminal and the non-inverting input terminals of the differential amplifier 11 to V B  (e.g., 3 volts). As a result, 3 volts is applied to the floating diffusion region 10. Next, the reset signal RST is set to the &#34;H&#34; level to render the analog switch 14 non-conductive, whereby the potential under the floating diffusion region 10 is initialized to a level corresponding to 3 volts. At the time instant T1 of the initialization, as shown in FIG. 4B, the signal charge q 1  closest to the output side is transferred to the region under the gate electrode 8, and the next signal charge q 2  is transferred to the region under the gate electrode 4. 
     Next, at the time instant T2 the signal φ 1  is raised to the &#34;H&#34; level, the signal φ 2  is set to the &#34;L&#34; level, and the signal φ 2A  is set to the &#34;L&#34; level in the negative range, so that the regions under the gate electrodes 7 and 8 are reduced in potential as shown in FIG. 4C. As a result, the signal charge q 1  is transferred over the potential barrier under the gate electrode 9 to the floating diffusion region 10, and the signal charge q 2  is transferred to the region under the gate electrode 6. Hence, the signal charge q 1  thus transferred charges the capacitive element 13, so that the output signal SC 1  of the differential amplifier 11 changes as shown in FIG. 6. 
     Next, at the time instant T3, the sample and hold signal SH is set to the &#34;L&#34; level. Hence, the output signal SC 1  of the differential amplifier 11 is held by the capacitive element 17, and an output signal S 0  proportional to the signal thus held is provided at the output terminal 15 by the differential amplifier 12. 
     At the time instant T4, the same operations are carried out as at the time instant T1, the next signal charge transferred next is read, and similarly the process for the period τ is performed. Thus, signal charges which are successively transferred are read out. 
     As described above, in the CCD delay line, the output stages of the switched capacitor integrator and the buffer amplifier in the output circuit are implemented with the CCD amplifier circuit according to the invention. Hence, the output circuit is low in output impedance and wide in bandwidth thus being suitable for processing video signals. 
     As described above, with the CCD amplifier circuit of the invention, even when the output signal increases in voltage amplitude, neither is the mutual conductance of the active load MOS transistor decreased nor is the pinch-off voltage decreased because a forward bias voltage is always applied between the gate and source electrode of the active load MOS transistor. Furthermore, the gate capacitance of the active load MOS transistor is charged by the control signal of low output impedance provided by a control circuit, and thus the active load MOS transistor does not become a capacitive load for the driving MOS transistor. Hence, the waveform of the output signal does not collapse due to the effect of the pinch-off voltage because the dynamic range is increased. In addition, the output impedance is not increased, and the high-frequency cut-off frequency can be increased. Hence, the resultant CCD amplifier circuit has a wide bandwidth. 
     Furthermore, a CCD delay line capable of processing video signals over a wide frequency range can be formed by applying the inventive CCD amplifier to its output circuit. 
     While the invention has been described in connection with the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is intended, therefore, to cover in the appended claims all such changes and modifications which fall within the true spirit and scope of the invention.