Page 238 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Oscillator SignaL (Peak Sinusoidal Voltage) g . /g for Any I
. m_conversIon . mQCQ
0.013 0.242
0.026 0.446

0.052 0.698
0.078 0.810
0.130 0.893

0.182 0.926
0.260 0.949
As can be seen in Table 14-1, there is a law of diminishinQi returns, and generally,
an oscillator signal of at least 100 mV peak to peak will provide sufficient drive.

Table 14-2 shows the conversion transconductance at 1 mA DC collector current.
TABLE 14-2 Conversion Transconductance at 1 mA as a Function of Oscillator
Drive Voltage

Oscillator SignaL (Peak Sinusoidal Voltage) 9m3 0nversion@ 1 mA Ica in mho or S

0.013 0.0093
0.026 0.0171
0.052 0.0268

0.078 0.0311
0.130 0.0343

0.182 0.0356
0.260 0.0364
Since the conversion transconductance gm_conversion is proportional to the DC

collector current, the numbers in the right column of Table 14-2 can be scaled
appropriately. For example, to find gm_conversion at 2 mA, just multiply the numbers
by 2, and to find gm_conversion at 100 ~A, just divide the numbers by 10. Again, the
values for gm_conversion are only for output signals that provide frequencies of Fl - F2

or Fl + F2.
For a quick example, suppose that the oscillator injects a 78-mV peak sine wave
into the mixer. What would be the conversion gain for an IF tank circuit with an
equivalent parallel resistance R = 150 k

when the DC collector current is 100 ~A?

The conversion gain is gm_conversion X R. At 1 mA for 78 mV peak,

so 100 IJA is one-tenth of 1 mA, which leads to