COMPLETE PIECES OF TRANSISTOR APPARATUS 



due to Potok and Wales'. This is a voltmeter of sensitivity of 1 MQ/V. 

 On applying the voltage to be measured, V, at the input terminals, a 

 current V/R flows out of the base of the transistor, causing an increase in 

 collector current §{VIR) which is indicated by the meter. A is the main 

 collector supply battery. In the absence of any test voltage an emitter-to- 

 collector leakage current (1 + ^)Ic(o) flows, and this is 'backed-off' on the 

 meter by current from an auxiliary battery via the associated variable 



S«t2ero 



-o- 



■AAAAA 

 R 







♦ o- 



50nA 



Fsn 



1- 



Figure 45.32 



resistor, which forms the 'set-zero' control. Due to the temperature 

 dependence of /^(o), a different zero setting will be frequently required. 

 Zero-drift with temperature change can be offset by using two transistors 

 in a push-pull circuit. Figure 45.33 shows an example due to Johnson*. 

 The arrangement converts a 0-50 ^A movement into an apparatus having 

 full-scale deflection for 2 ^A. Unfortunately there is nothing in this type 

 of circuit to prevent the rise of collector current with rising temperature. 

 Such a rise may take the transistor working points outside their working 

 regions, so that correct amplification cannot take place. 



Figure 45.33 



We have seen that the inclusion of resistance in the emitter circuit serves 

 to off"set the effects of temperature. We have also seen that such a resistance 

 introduces negative feedback and reduces gain. In a.c. coupled circuits, 

 where there is a definite minimum frequency at which the circuit is required 

 to operate, this feedback may be eliminated by shunting the emitter resistance 

 with a large capacitance ; in direct-coupled circuits there is no such frequency 

 and another method is necessary. 



The technique is to use the transistor equivalent of the long-tailed pair 

 in valves {Figure 45.34). This is a differential current amplifier which will 

 discriminate against in-phase input signals. Inputs in opposite phase 

 produce equal and opposite changes in collector current; there is no net 

 change in the current through the common emitter resistance, and no 



46A 697 



