Page 133 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 9

Low-Power Superheterodyne Radios

Chapter 8 presented superheterodyne radios of moderate current drain (e.g., >10
mA) that powered loudspeakers. This chapter will explore superheterodyne
receivers with extremely low current draw that allow years of continuous playing on
a single battery. What we will find out is that the superheterodyne circuit topology
shown in Chapter 8 will not apply for low-power design, and instead, a modified

circuit topology is required.
Design Goalis for Low Power

The design goals are as follows:
1. Current drain of Iless than 150 IJA with a crystal earphone
2. Operational supply voltage of 1.2 volts

3. Sensitivity and selectivity performance at least equal to that of the
four-transistor design in Chapter 8
As with the low-power circuits in earlier chapters, the goals in terms of current
drain and supply voltage remain the same. However, because the circuit topology
includes an oscillator, mixer, and two intermediate-frequency (IF) amplifiers, one
would think that power consumption would go up. Instead, we design these

circuits, the mixer, and IF amplifier to run each at about 20 IJA or less. Moreover, a
low-power superheterodyne radio with less than 150 IJA of drain will last about 5
years continuously on a single alkaline C cell.
Low-Power Oscillator, Mixer, and

Intermediate-Frequency Circuits

In Chapter 8 we found out that with commercially made oscillator coils such as the
42IFIOO or 42IF300, the oscillator or converter transistor's collector current
requires about 200 IJA to ensure reliable oscillation over the tuning range of 1 MHz

to 2 MHz. The step-down ratio of the oscillator coil thus requires a minimum
transconductance or gain from the transistor for oscillation to occur.
Transconductance of a bipolar transistor is roughly equal to the direct-current (DC)
collector current divided by 0.026 volt. For example, if the DC collector current is 1
mA, then the transconductance of the transistor is 0.001mA/0.026 volt = 38
mA/volts. As you can see, transconductance is proportional to DC collector current.
For a low-power oscillator, the transconductance of the transistor will be much

lower than the transconductances of the OSCillator/converter transistors shown in
Chapter 8. Therefore, the OSCillator/converter circuits will not oscillate reliably at DC
collector currents of less than 100 IJA.