Page 214 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The output voltage then will be about 156 mV x 0.312 x 0.038 x 2,700 = 5.0 volts

peak. Note that 0.312 x 0.038 = G ml the large-signal transconductance, which is
smaller than the small-signal transconductance by a multiplying factor of about
0.312.
The Q of the tank circuit is determined primarily by the load resistor RL = 2,700;
that is,

L
The actual capacitance is really smaller because Cl is in series with C2. But
because C2 » Cl, Cl can be used in the calculation. So the Q of the circuit is
about 30 (give or take 10 percent).

In general, the actual output will be smaller than the predicted output voltage value
because the inductor has losses and the transistor includes series emitter and base
resistors that reduce its transconductance. For example, with the one-transistor
oscillator with the values R1 = 2,700

, RE = 1,000

, Cl = 0.0018 IJF, C2 = 0.056 IJF, and L1 = 15 IJH, the actual output was smaller
by at least 30 percent. That is, instead of the 5.0 volts peak, fewer than 3.5 volts
was provided with a standard fixed inductor and a 2N3904 transistor. It turns out
that at 1 MHz, the 15-~H inductor is very lossy and thus has a low Q that lowers
the output voltage. By maintaining Av = 3.2 by using the same values for R1 =

2,700

and RE = 1,000

and raising the frequency to about 3 ,MHz by dividing the capacitor values by 10

(i.e., Cl = 180 IJF and C2 = 0.0056 pF) and replacing the 2N3904 transistor with a
2N4401, the output increased to about 4.25 volts peak, which is within about 15
percent of the calculated 5.0 volts peak.
It should be noted that the input impedance at the emitter of the transistor is often
low enough to cause additional signal loss, which thereby lowers the loop gain of
the oscillator. This extra lowering of the loop gain results in a lower oscillator

output signal. Normally, C2 is made sufficiently large to load into the emitter with
minimal signal loss.
Differential Pair Oscillator

Figure 13-10 shows a differential-pair oscillator circuit that is similar to the ones
shown in Figures 9-2 and 9-4 of Chapter 9. Essentially, this circuit has the same
elements as the one-transistor oscillator circuit with the exception of an extra