TRANSISTORS 



feedback. The larger R,, the better the stabilization, but to preserve a given 

 emitter-collector voltage and current for the transistor, R^ and the positive 

 voltage to which Rg is returned must be increased pro rata. Neale and 

 Oakes have pointed out^ that this may lead to unnecessarily high battery 

 voltages, and described a circuit in which an additional transistor was used 

 to provide stabilizing action. In this manner an amplifier was constructed 

 having a gain of 45 dB's and a zero stability corresponding to 10~^ amps 

 at the input. Two transistors in push-pull earthed emitter fed a further 

 pair in earthed collector; the latter were used to drive a meter. A fifth 

 transistor was arranged to stabilize the first stage transistor at a low-noise 

 working point (collector voltage between —0-2 and —1 V). 



Input 1 



put 2 



Figure 45.34 



Ret. wave in 



Figure 45.35 



It is interesting to consider the possibility of using apparatus such as that 

 of Neale and Oakes as a pre-amplifier for use with micro electrodes in 

 electrophysiology. If the zero stability is 10~^ amps, then if the amplifier is 

 fed from a 20 MQ microelectrode, the corresponding noise voltage at the 

 electrode tip is 20 mV. The penetration of a cell (70 mV input or thereabouts) 

 could probably be detected, but the device is likely to be too noisy for the 

 extracellular recording of discharges from neighbouring cells. 



Carrier amplifiers — There is no difficulty, in principle, in making a 

 transistorized carrier amplifier merely by interposing an a.c. coupled 

 transistor amplifier between a relay chopper and a relay phase-sensitive 

 rectifier; yet this is seldom done. An important characteristic of transis- 

 torized devices is their small size, and there is little to be gained by using 

 a very small amplifier if it works between two large relays. Consequently, 

 designs for carrier amplifiers employing transistors involve chopper and 

 rectifier devices of smaller proportions. 



Some of these invoke the almost perfect performance of the recently 

 developed silicon signal diode. We have seen in Chapter 5 how diode 

 networks can be used for phase-sensitive rectification; the same circuits 

 can also be used to chop. For example, the arrangement in Figure 45.35 

 (which, if R is replaced by a capacitance, we have hitherto regarded as a 

 quasi-Cowan bridge rectifier) can clearly be used as a chopper. When the 

 reference wave transformer drives current clockwise round the diode circuit, 

 the diodes are switched to low resistance and the output is short-circuited. 

 When the reference wave reverses, the diode resistances become very high. 

 The magnitude of the output depends on the input, and also on the ratio 



698 



