312 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



ture. Consequently, the base current of the first stage, Ih , and the output 

 voltage of the amplifier must change with temperature in order to main- ' 

 tain Ic constant. The drift due to the temperature variation in a can be 

 reduced by operating the first stage at a low value of collector current. 

 With a germanium junction transistor in the first stage operating at a 

 collector current of 0.25 milliamperes, the output voltage of the amplifier 

 drifts about ±1.5 volts over a temperature range of 0°C to 50°C. It is 

 possible to reduce the dc drift by using temperature sensitive elements 

 in the amplifier. • In general, temperature compensation of a transistor 

 dc amplifier requires careful selection of transistors and critical adjust- 

 ment of the dc biases. However, even with the best adjustments, tem- 

 perature compensation cannot reduce the drift in the amplifier to within 

 typical limits such as ±5 millivolts throughout a temperature range of i 

 to 50°C. In order to obtain the desired accuracy it is necessary to use 

 an automatic zero set (AZS) circuit. t 



Fig. 11 shows a dc summing amplifier and a circuit arrangement fori 

 reducing any dc drift that may appear at the output of the amplifier. 

 The output voltage is equal to the negative of the sum of the input volt- 

 ages, where each input voltage is multiplied by the ratio of the feedback 

 resistor to its input resistor. In addition, an undesirable dc drift voltage ^ 

 is also present in the ovitput voltage. The total output voltage is 



^o.t = -i:^y|^ + Adrift (1!))^ 



In order to isolate the drift voltage, the A^ input voltages and the output 

 voltage are applied to a resistance summing network composed of re- 

 sistors Ro , Ri , R2 , • • • , Rn ■ The voltage across Rs is equal to 



Es=^ Adrift (20) 



if 



R,«Ro,R/; j = 1,2, ■■' ,N 

 and 



RoRj = RkR,'; j = 1,2, ■■■ ,N 



The voltage E, is amplified in a relatively drift-free narrow band dc 

 amplifier and is returned as a drift correcting voltage to the input of the 

 dc summing amplifier. If the gain of the AZS circuit is large, the drift 

 voltage at the output of the summing amplifier can be made very small. 

 Fig. 12 shows the circuit diagram of a summing amplifier which uses 

 a mechanical chopper in the AZS circuit.^^ The AZS circuit consists of a 



