ble-shoot a defective radio.
When SW1 is switched to TRACE position 2, the
Figure 3 circuit is configured as a cascaded pair of common-emitter amplifiers, with the probe input feeding to
Q1 base, and Q2 output feeding into an earpiece or
head-set. Any weak audio signals fed to the probe are
directly amplified and heard in the earpiece, and any
amplitude-modulated RF signals fed to the probe are
demodulated by the non-linear action of Q1. The resulting audio signals are then amplified and heard in the earpiece. By connecting the probe to suitable points in a
radio, the tracer can thus be used to trouble-shoot a
faulty radio, etc.
Figure 5. An npn current
mirror.
Figure 6. A pnp current
mirror.
LIE DETECTOR
Figure 7. Adjustable zener.
The lie detector of Figure 4 is an
experimenter's circuit, in which the victim is connected (via a pair of metal
probes) into a Wheatstone bridge,
formed by R1-RV1-Q1 and R3-R4; the
1 mA center-zero meter is used as a
bridge-balance detector. In use, the victim makes firm contact with the
probes and, once he/she has attained
a relaxed state (in which the skin resistance reaches a stable value), RV1 is adjusted to set a null on the meter. The
victim is then cross-questioned and, according to theory,
the victim's skin resistance will then change, causing the
bridge to go out of balance if he/she lies or shows any sign
of emotional upset (embarrassment, etc.) when being
questioned.
AN
ADJUSTABLE
ZENER
Figure 8. MW signal
generator/BFO.
CURRENT MIRRORS
A current mirror is a constant-current generator in
which the output current magnitude is virtually identical to
that of an independent input control current. This type of
circuit is widely used in modern linear IC design. Figure 5
shows a simple current mirror using ordinary npn transistors; Q1 and Q2 are a matched pair and share a common
thermal environment.
When input current Iin is fed into diode-connected Q1,
it generates a proportionate forward base-emitter voltage, which is applied directly to the base-emitter junction
of matched transistor Q2, causing it to sink an almost
identical (mirror) value of collector current, Isink. Q2 thus
acts as a constant current sink that is controlled by Iin,
even at collector voltages as low as a few hundred millivolts.
Figure 6 shows a pnp version of the simple current
mirror circuit. This works in the same basic way as already
described, except that Q2's collector acts as a constant
current source that has its amplitude controlled by Iin. Note
that both of these circuits still work quite well as current-controlled, constant-current sinks or sources, even if Q1
and Q2 have badly matched characteristics, but in this
case may not act as true current mirrors, since their Isink
and Iin values may be very different.
FEBRUARY 2004
Figure 7 shows the circuit of an adjustable zener that
can have its output voltage pre-set over the range 6. 8 V
to 21 V via RV1. The circuit action is such that a fixed reference voltage (equal to the sum of the zener and Vbe
values) is generated between Q1's base and ground
(because of the value of zener voltage used) and has a
near-zero temperature coefficient. The circuit's output
voltage is equal to Vref multiplied by (RV1 + R1)/R1, and
is thus pre-settable via RV1. This circuit is used like an
ordinary zener diode, with the RS value chosen to set its
operating current at a nominal value in the range 5 to 20
mA.
L-C OSCILLATORS
L-C oscillators have many applications in test gear and
gadgets, etc. Figure 8 shows an L-C medium-wave (MW)
signal generator or beat-frequency oscillator (BFO), with
Q1 wired as a Hartley oscillator that uses a modified 465
kHz IF transformer as its collector load. The IF trans-former's internal tuning capacitor is removed, and variable
oscillator tuning is available via VC1, which enables the
output frequency (on either fundamentals or harmonics)
to be varied from well below 465 kHz to well above 1.7
MHz. Any MW radio will detect the oscillation frequency if
placed near the circuit; if the unit is tuned to the radio's IF
value, a beat note will be heard, enabling CW and SSB
transmissions to be clearly heard.
Figure 9 shows the above oscillator modified so that,
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