The transistor base--emitter voltage drop (VBd introduces an error in the simple
emitter-follower voltmeter. For example, when E is 5 V in the circuit in Example 4-1, the
meter should read half of full-scale, that is, 0.5 rnA. However, as a simple calculation
shows, the meter current is actually 0.46 rnA. The error can be eliminated by using a potential
divider and an additional emitter follower, as illustrated in Figure 4-2.
The practical emitter-follower circuit in Figure 4-2 uses a plus-and-minus. or dualpolarity
supply (typically, ±12 V). Transistor Q has its base biased to ground via resistor
Rio and a potential divider (R4, Rs. and R6 ) provides an adjustable bias voltage (Vp) to the
base of transistor Q2' Resistors R2 and R3 connect the transistor emitter terminals to the
negative supply voltage (-V£d, and the meter circuit is connected between the transistor
emitters. The circuit input resistance is R in parallel with the input resistance at the transistor
base.