Page 220 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

TABLE 13-4 Ratio of Large- to Small-Signal Transconductance as a Function of

Sinusoidal Input Level

Input Signal (Peak Sinusoidal Voltage) Gjgm for Any lEE

0.001 1.000
0.026 0.950
0.052 0.825

0.078 0.675
0.104 0.560

0.130 0.475
0.156 0.410
0.182 0.360

0.208 0.310
0.234 0.280

0.260 0.250
0.286 0.230
0.312 0.210

0.338 0.195
0.364 0.185

* As read off a graph from Communication Circuits: Analysis and Design, by Kenneth K. CLarke and Donald T.
Hess (Addison-Wesley, 1971).
For designing a 3-MHz differential-pair oscillator as seen in Figure 13-10, consider
the foUowing: Given that L1 = 15 IJH, Cl = 180 pF, C2 = 1,620 pF, and Q = 10,

find Rl, the emitter tail current lEE and voltage output at the collector of Q1. Note
that Cl/(Cl 1 C2) = 1/10.
The series capacitance of Cl and C2 results in 162 pF. And 162 pF with the 15-IJH
inductor L results in a resonant frequency of 3.23 MHz. Therefore, Q = 2n(3.23
MHz) RL (162 pF) = 10. Solving for Rl yields R1 = 3,000

for the closest = percent-valued resistor.
First, pick an initial small-signal loop gain of 3 to solve for the tail current lEE:

x . X 1 I / . 2 It = ~ / . 04)
2 4 1
where lEE = 1.04 mA ~ 1 mA. Because the initial small-Signal loop gain = gm/Gm =

3,