The comparatively low collector to emitter voltage ratings of transistors limit their maximum output voltage swing.

 

However, the transistor amplifier shown here can operate with an output voltage swing of 300-400 v.
Fig. 1 shows the basic series arrangement with which the high output is obtained.
A current injected at the base of the top transistor must flow down the series string, increasing the drop across RL and decreasing the drop across each transistor in the string. The decreased drop across each transistor is accompanied by an increase in each base current through the series base resistors. If the series base resistors are chosen so that all base current are always equal, then the expression for the output collector current is i_L = ((β — n) i_b0,
where i_L = current through RL
β = current gain of each transistor
n = number of transistors in the series
string
i_b0 = base current at input
If the injected current is removed, then each transistor is cut off and the drop across each is determined by the bleeder chain, R_B.

The circuit of Fig. 1 is therefore equivalent to one transistor of nV_LCEmax collector-emitter rating with a current gain of (β — n).
For purposes of experiment, a 300-v supply was chosen and ten transistors (type 2N247, 35-v collector-emitter rating, β = 60) were connected in series, Fig. 2. With R_f open, this circuit is equivalent to one transistor with a 350-v collector rating and an opimum β equal to 50. R_f was added for stabilization. With R_f in the circuit the amplifier had a gain of ten, a useful output voltage swing approaching 300 v, and a frequency response flat from dc to 0.5 mc.
Linton S. Kypta, University of Illinois, Urbana, III