DUALITY AS GUmE IN TRANSISTOR CIRCUIT DESIGN 399 



the feedback is positive. As the amplitude of oscillation increases, Rt' is 

 heated so that its resistance decreases and brings the bridge more nearly 

 into balance. This reduces the amount of positive feedback until a stable 

 amplitude of oscillation is reached with Rt' only a little greater than Rx'. 

 Meacham has shown that the stabihty of such an oscillator increases as 

 the gain of the amphfier is increased. Since the vacuum tube amplifier of 

 Fig. 7(a) can be made to give more gain than can be obtained from a single 

 transistor, the transistor oscillator of Fig. 7(b) is not as stable as its vacuum 

 tube dual. If increased stabihty is desired, it can be obtained by using a 

 two-stage transistor amplifier instead of the single transistor shown. 



Circuits Using Vacuum Tubes and . Transistors Together 



Since the vacuum tube and the transistor are basically different kinds of 

 circuit elements, it seems reasonable to suppose that there may be circuits 

 in which both can be used together to advantage. Two examples of such 

 circuits will be discussed. The first has to do with a very ingenious high 

 efficiency linear amplifier designed by Mr. W. H. Doherty.^ This amplifier 

 is particularly suited for use with ampHtude modulated radio frequency 

 inputs. 



Figure 8 shows the basic features of one form of the Doherty amplifier. 

 The networks iVi and iV2 are impedance inverting networks of the type 

 already discussed and amount to ideal gyrators for frequencies near the 

 carrier frequency. Tube Ti is biased nearly to cutoff and works, for small 

 rf inputs, as a linear class B amplifier; while T2 is biased well below cutoff 

 and is inactive except when the rf input is higher in level than the unmodu- 

 lated carrier. Downward swings of modulation are amplified by Ti alone, 

 which sees an effective load impedance just twice the value into which it 

 could deliver maximum power. Under these conditions the peak voltage 

 swing of Ti begins to approach the supply voltage just as the rf input reaches 

 a value corresponding to the unmodulated carrier. For greater input signals 

 Ti , if acting alone, would begin to distort. But as the input signal is increased 

 above the value corresponding to the unmodulated carrier, T2 comes into 

 action and contributes in two different ways to increasing the output signal 

 Hnearly. First, T2 acts as a class C amplifier and delivers power to the load 

 and second, through the action of the impedance inverting network N2 , 

 T2 acts in such a way as to lower the effective load impedance seen by Ti . 

 This makes it possible for Ti to deliver more power to the load without an 

 increase in plate voltage swing. The result of all this, which is discussed in 

 much greater detail in Doherty's papers, is a Hnear amplifier of unusually 

 high efficiency. 



*A New High-Efficiency Power Amplifier for Modulated Waves, W. H. Doherty, 

 Proc. I.R.E., 24, 1163 (September, 1936). 



