The present invention relates to a current mirror amplifier (CMA) with improved frequency response which is especially beneficial when implemented in CMOS technology.
A current mirror amplifier is a current amplifier with a current gain of minus unity commonly used in integrated circuitry. Current variations applied to its input circuit, which customarily exhibits relatively low impedence, will cause corresponding current variations equal and opposite thereto in its output circuit, which customarily exhibits relatively high impedence.
Current mirror amplifiers are useful in various applications. Current mirror amplifiers are often employed as active loads for differential amplifiers. They are also used in operational transconductance amplifiers (OTA's), which are becoming popular for switched-capacitor applications.
A common problem in MOS amplifiers is the presence of a low-frequency right-half plane zero. This zero is present in bipolar circuits as well, but is typically at a high frequency due to the higher transconductance of bipolar devices.
Voltage amplifier performance for alternating current (ac) operation can be estimated and designed for by inspection of the poles and zeros of the gain function, where gain=V.sub.out /V.sub.in. Poles represent the values of the complex frequency, s, at which the gain of the amplifier is theoretically infinite. Zeros represent the values of the complex frequency at which the gain of the amplifier is equal to zero. The range of usable frequency values for the operation of the amplifier is related to the bandwidth of the amplifier. The 3 dB bandwidth is the frequency at which the gain has fallen by 3 dB to approximately 70% of its low-frequency value.
When designing amplifier circuits, a technique commonly used is to substitute a small signal model of the transistor for ac analysis. (Small signals represent values at which the transistor never operates very far from its DC operating point.) At low frequencies, the capacitive reactance, looking into the gate of an MOS transistor, is so large in comparison with other impedences in the circuit that its input may be considered to be an open circuit. Therefore, MOS transistor Q7 shown in FIG. 1A, can be modelled as an ideal voltage-controlled current source with transconductance equal to g.sub.m, as shown in FIG. 1B. The transconductance is: EQU g.sub.m =i.sub.d /v.sub.gs
where i.sub.d is the drain current and v.sub.gs is the gate-to-source voltage.
When transistor Q6 is used in a current mirror circuit its gate is tied to its drain by a connecting wire 1, as shown in FIG. 1C. The small signal model for analysis of this arrangement is shown in FIG. 1D. With this connection, the input current and resistance are described by: EQU i.sub.in =i.sub.d6
and EQU z.sub.in =v.sub.in /i.sub.in =v.sub.gs /i.sub.d6 =1/g.sub.m6.
An example of a prior art MOS current mirror circuit, constructed with the component circuits shown in FIGS. 1A and 1C, is shown in FIG. 2. Operational amplifiers incorporating such prior art circuits are characterized by a first stage mirror pole present in one-half of the signal path, with another pole occurring at a higher frequency in the other half of the signal path. The mirror pole frequency=g.sub.m6 /2.pi.(C.sub.g6 +C.sub.g7), where g.sub.m6 is the transconductance associated with transistor Q6, and C.sub.g6 and C.sub.g7 are the gate-to-source capacitances associated with transistors Q6 and Q7, respectively. The first stage mirror pole contributes excess phase shift which degrades the stability of operational amplifiers in closed-loop applications by reducing phase margin.