(a) Field of the Invention
The present invention relates to amplifiers, and more specifically amplifiers with substantially no distortion wherein distortion generated in the amplifier is detected and added to an input signal to be amplified, for cancellation of distortion in such amplifier.
(b) Description of the Prior Art
It is known in the art that an input signal of such as class-B amplifiers under a fixed bias operation is subjected to distortion if the output signal is assumed to be free of distortion.
Shown illustratively in FIG. 1 is an amplifier circuit 7 of a class-B push-pull type, wherein NPN and PNP type transistors 3 and 4 with emitter resistors 5 and 6 (having the same value R.sub.E) are supplied with bias current from respective bias sources 1 and 2. The relation between the emitter current I.sub.E (or collector current I.sub.C) and the base-emitter voltage V.sub.BE of each transistor is shown in FIG. 2 as curves a and b which have exponential characteristic function. The curve a is representative of a relation between the emitter current I.sub.E (whose value is I.sub.E1) and the base-emitter voltage V.sub.BE (whose value is V.sub.BEN) of the transistor 3, while the curve b is representative of a relation between the emitter current I.sub.E (whose value is I.sub.E2) and the base-emitter voltage V.sub.BE (whose value is V.sub.BEP) of the transistor 4. A straight line c is also shown in FIG. 2 which indicates an ideal linear relation between the emitter currents and the base-emitter voltages. Assuming that the output current I.sub.0 appearing at an output terminal 8 has no distortion and changes by .DELTA.I.sub.0, each of the base-emitter voltages V.sub.BEN and V.sub.BEP is changes by .DELTA.V.sub.BEN and .DELTA.V.sub.BEP, respectively, and is distorted due to the above-mentioned exponential characteristics of the transistors 3 and 4. Therefore, the voltage e.sub.i i of an input signal 10 at an input terminal 9 is distorted by (.DELTA.V.sub.BEN -.DELTA.V.sub.BEP). Further, in this case, the voltage R.sub.E.I.sub.E1 and R.sub.E.I.sub.E2 delivered across each emitter resistor 5 and 6 of the respective transistors 3 and 4 are also distorted due to the exponential characteristics. As a result, assuming that when the output current I.sub.0 changes by .DELTA.I.sub.0, the emitter currents I.sub.E1 and I.sub.E2 are changed by .DELTA.I.sub.E1 and .DELTA.I.sub.E2, respectively, and that each current gain h.sub.FE of transistors 3 and 4 is linear, the input current I.sub.i of the input signal 10 is distorted by .DELTA.I.sub.E1 /h.sub.FE and .DELTA.I.sub.E2 /h.sub.FE. Thus, both the input voltage and current e.sub.i and I.sub.i are subjected to distortion.
It is customary in the art to employ a Darlington configuration to each transistor 3 and 4 to perform the distortion reduction of the driving current I.sub.i. Apart from this, in order to reduce the distortion of the driving voltage e.sub.i, it is necessary to provide charging and discharging currents for the parallel capacitance at the developing portion of the distortion, that is, at the inputs of each transistor 3 and 4. Charging and discharging currents through the capacitance, however, cause to distort the driving current I.sub.i. More concrete description of such a case is given with reference to FIG. 3, wherein a main portion of the amplifier circuit 7 is illustrated with the transistor 3 is FIG. 1 replaced by three transistors 3a through 3c in a Darlington connection. In the amplifier circuit 7, it is assumed that the value of a capacitance 11 produced between the base of the transistor 3a and ground is 47 pF, the value of a load resistor 12 connected between the output terminal 8 and ground is 8.OMEGA., and the current gain h.sub.FE of each transistor is 100. At this condition, if the peak value of the output current I.sub.0 of 10 A at 100 kHz is desired, then the peak value of the base current I.sub.b of the transistor 3a is given by ##EQU1## If the distortion voltage of 1 V is assumed to be generated between the base of the transistor 3a and the emitter of the transistor 3c, the charging current Id forced to flow through the capacitance 11 is given by ##EQU2## From the above equations (1) and (2), it is noted that the charging current Id becomes larger than the base current Ib. Therefore, it is important for such an amplifier circuit to eliminate, at first, voltage distortion rather than to increase the number of the transistors to be connected in a Darlington fashion. Thus, it is desirable, in order to reduce both voltage and current distortion, to eliminate voltage distortion prior to eliminating current distortion.
One of the prior art techniques for reducing voltage and current distortion is the application of negative feedback. Negative feedback, however, has some deficiencies in its nature. There are limits to the degree of reduction of distortion such that the reduction degree charges in accordance with frequency, and in the limit at the gain crossing frequency or cut-off frequency there will be absolutely no improvement in distortion reduction. Moreover, since the feedback signal containing a distortion component passes throughout the feedback loop, irrespective of the location where distortion is originally generated, intermodulation distortion occurs between the signal to be amplified and the feedback signal. Again, in negative feedback, feedback signals have a comparable level to the signal to be amplified because the feedback signals contain not only a distortion component but also the signal to be amplified, so that the stability of the amplifier depends to a large extent on the transfer, i.e., frequency and phase, characteristics of the open gain of the amplifier. This results in a limited frequency band of negative feedback and a limited feedback amount, and hence there are limits to the distortion reduction effect.