830 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



0.01 0.02 



0.05 0.1 0.2 

 FREQUENCY 



0.5 1.0 2 

 IN MEGACYCLES 



5 10 20 

 PER SECOND 



50 100 



Fig. 11 — Measured and computed gain of a common base video amplifier. 



amplification is obtained through the ratio of impedances of load to 

 source. 



Since it is impossible to match the output impedance for maximum 

 gain due to reasons outlined above, Equation 5 of Fig. 1 can be used to 

 compute the gain once the load impedance is determined. If we use a 

 load impedance of 2,000 ohms and a source impedance of 75 ohms, the 

 difference between the computed gain using the approximate of Fig. 10 

 and the exact expression (Equation 5 of Fig. 1) amounts to less than 1 

 db at frequencies up to 10 mc. At 30 mc, the exact expression results in 

 a computed gain 1.5 db lower than that obtained from the approxima- 

 tion. A comparison of measured and computed gain for a common base 

 video amplifier is shown on Fig. 11. Using a resistive load of 2,000 ohms 

 a gain of 14.5 db is obtained at low freciuencies and the response is down 

 3 db at 17 mc. To equalize the decreasing | h-n \ with frequency and the 

 increasing effect of the capacitance, a load consisting of an inductor and 

 a resistor is used. The circuit is shown on Fig. 12 and it ^^•ill be noted 

 from the response on Fig. 11, that the low frequency gain is 14.5 dl) with 

 the 3 db point occurring at about 26 mc. 



Common base stages can, of course, be cascaded to advantage only if 

 impedance transformation is provided in the interstage coupling. Prac- 

 tical transformers or coupling networks may introduce undesirable band 

 limitation. In the next section we will consider common-emitter stages 

 which can be cascaded without impedance transformation. 



Common Emitter 10-Mc Video Amplifier 



To get a first idea of feasible impedance levels for a common-emitter 

 video amplifier, one recognized from Table I that the input impedance 



!1 



