THE DESIGN OF TETRODE TRANSISTOR AMPLIFIERS 827 



do not ordinarily constitute a finished solution, however, since practical 

 constraints frequently modify the design used. Moreover, all of the 

 relationships are expressed on a single frequency basis, and many times 

 the amplifier must operate over a range of frequencies broad enough that 

 parameters change significantly over the range. 



Four amplifier designs are described in this section: a single stage, 

 common-base, 20-mc ^ddeo amplifier; a common-emitter, 10-mc video 

 amplifier; an IF amplifier at 30 mc and a 60 to 80-mc IF amplifier. 

 Parameter measurements made with bridges support the first three 

 designs. 



Parameter values and associated constants of a typical tetrode 

 transistor are given in Table I. The quantities shown there reveal some 

 interesting facts about the typical tetrode transistor represented. First, 

 in the common-base connection the tetrode is potentially unstable at 

 30 mc but not at the lower fre(|uencies. The common-emitter amplifier 

 is potentially unstable at 1 and 3 mc. Second, the power gains of common- 

 emitter and common-base stages are about the same at 30 mc, the com- 

 mon-emitter connection giving more gain at low frequencies. 



The matter of potential instability requires further consideration from 

 a practical point of view. Potential instability at a frequency neither 

 implies that a stable amplifier cannot be built at that particular fre- 

 quency, nor does it imply that one can obtain an unlimited amount of 

 stable amplification at that frequency. It does mean that by simul- 

 taneously tuning output and input one can adjust for oscillation. The 

 region of potential instability corresponds to a region in which the input 

 resistance may be negative for appropriate loads. Instability is avoided 

 in the physical amplifier if one supplies the amplifier from a sufficiently 

 high impedance that the input loop impedance always has a positive 

 real part. To operate the amplifier Avith such a load that it presents a 

 negative resistance to the source is attended by the difficulty that the 

 amplification is more sensitive to changes in the source impedance than 

 it is when the input resistance is positive. Hence the possible higher gain 

 with internal positive feedback goes along with a greater sensitivity to 

 changing termination impedance. 



! A Common-Base 20-Mc Video Amplifier- 



The data presented in Table 1 gives a ciuite comprehensive picture 

 of possibilities for amplifier designs. To it must be added a practical 

 fact. It is difficult to connect the load impedance without adding about 

 2 jujuf of capacitance. This means that any termination considered must 

 include about this amount of capacitance. By a theorem regarding 



