Circuit Description The differential transistor pair Q and Q form a transcon- 4 5 ductance stage in that the ratio of their collector currents is (5) defined by the differential input voltage according to the The term in brackets is then the transconductance of the am- transfer function: plifier and is proportional to I . ABC Linearizing Diodes (1) For differential voltages greater than a few millivolts, Equa- where V is the differential input voltage, kT/q is approxi- tion (3) becomes less valid and the transconductance be- IN mately 26 mV at 25˚C and I and I are the collector currents comes increasingly nonlinear. Figure 1 demonstrates how 5 4 of transistors Q and Q respectively. With the exception of the internal diodes can linearize the transfer function of the 5 4 Q and Q , all transistors and diodes are identical in size. amplifier. For convenience assume the diodes are biased 3 13 Transistors Q and Q with Diode D form a current mirror with current sources and the input signal is in the form of cur- 1 2 1 which forces the sum of currents I and I to equal I ; rent I . Since the sum of I and I is I and the difference 4 5 ABC S 4 5 ABC is I , currents I and I can be written as follows: I + I = I (2) OUT 4 5 4 5 ABC where I is the amplifier bias current applied to the gain ABC pin. For small differential input voltages the ratio of I and I ap- 4 5 proaches unity and the Taylor series of the In function can be Since the diodes and the input transistors have identical ge- approximated as: ometries and are subject to similar voltages and tempera- tures, the following is true: (3) (4) (6) Collector currents I and I are not very useful by themselves 4 5 and it is necessary to subtract one current from the other. The remaining transistors and diodes form three current mir- rors that produce an output current equal to I minus I thus: 5 4 DS007980-8 FIGURE 1. Linearizing Diodes www.national.com 6
