INTERSTAGE COUPLINGS 



in 



behaviour is now limited by the filter made by Rj^ and Cq in series with C 

 {Figure 9.18). The turn-over frequency obtained will be comparable with 

 those in other systems. In Figure 9.19 I have summarized the findings of this 

 section. The curves show the form of the frequency response to be expected 

 from the various systems in coupled triodes and coupled pentodes. It is 

 clear that amphfiers are low-pass or band-pass devices. For a complete 

 amplifier comprising a number of couphngs in cascade the overall frequency 

 response is the sum of the frequency response of the several couplings. 



TRANSIENT RESPONSE OF INTERVALVE COUPLINGS 



If a direct coupled ampHfier be tested by applying a square wave of voltage 

 at the input, the output does not rise instantly to the final value because of 

 the imperfections in the high frequency response. The output will be of 

 the form of Figure 9.20. The performance of complete amplifiers is some- 



/ «T 



Test 

 wave. 



n = 1 



Amplifier 

 output - 



J V. 



Time 



n = 2 



Figure 9.20 



n:3 



Figure 9.21 



times given in terms of the time taken by the output voltage to reach, say, 

 95 per cent of the proper value. This would be called the 'rise time for 95 

 per cent'. If the amplifier is a.c. coupled there is not only a finite rise time, 

 but also 'sag' due to the non-existence of gain at zero frequency. If all the 

 RC couplings have an identical time-constant T, and if the pulse length / <^ T, 

 then if the ampHfier contains n RC couplings, the response sags exponentially 



t»T 



n--2 



n=l Figure 9.22 n-3 



with a time constant T if « = 1, and approximately exponentially with a 

 time constant T/n if « > 1 (Figure 9.21). When /^ T the response is quite 

 different. The response for « > 1 is oscillatory, as is shown in Graph 7 

 {Figure 9.22). 



158 



