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1992-10-12
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92 lines
General Purpose ┌─────────────┬───────── + 6 to 18 V
High Impedence Preamp │ │
/ /
\ 220K \ 6.8K
/ / +
│ ├─────┤(────── output
│ ┌─C──┘ 10 µF Z
Input 10 µF ┌─C──┴────────┤ 2N3094 about 4KΩ
Z o──────)├──────┬──────────────B┤ 2N3094 └─E─>┐
500Ω to + │ └─E─>┐ Q2 │
3KΩ / Q1 │ │
\ 10K │ │
/ 100K │ │
├─────────/\/\/────────────────────┤
- ───────┐ │ + │ │
│ ─┴─ 10 µF / / R2
│ ─┬─ \ 1K \ 1K
│ │ / /
└──────┴────────────────────┴─────────────┴───────────── -
Referring to the figure above, the circuit can be seen as a direct
coupled pair of 2N3094 transistors. This transistor is cheap, high
gain, fairly low noise, and very easily obtained. The Q2 transistor is
hooked up like any ordinary amplifier stage, but the base resistor that
normally goes from its base to ground has been replaced with another
transistor, Q1. This Q1 transistor varies the bias on Q2, so the
circuit is immune to heat effects. The way it's hooked up, if Q2 draws
more current, the voltage on R2 rises, turns on Q1 harder via the 100k
resistor connected to its base, and cancels out he inreased current in
Q2. The result is almost no change in current due to temperature
variations. The capacitor C2 prevents the AC signal from being fed
back and reducing the overall gain. By placing the capacitor as shown,
a very small value, which is also small in physical size and cheaper,
will permit the amplifier to keep ites full gain to low frequencies as
well as would be the case for a very large C placed across R2. The
values in the figure above will amplify down to about 10 cycles using a
physically small capacitor. To make the amplifer roll off at a higher
frequency on the low side, reduce C2 to about 1 uF or less, or,
alternately you could reduce the 100k resistor to about 10k. This
would make the frequency roll off around 100 cycles and turn the
circuit into a speech amplifier rather than a hi-fi type.
The circuit shown above performs best when driven by a moderate
impedance source from 500 ohms to 3k ohms impedance. With this kind of
source, the gain will be about 250, and the output noise with no
signal in will be about 2 millivolts. This is equivalant to an input
noise of only 8 microvolts, so the noise is quite low for all but
extraordinary uses.
If you wish to drive the circuit with a low impedance source, such as
a speaker of 4 to 16 ohms or a telephone earphone (which makes an
excellent high output mike), use the circuit in the firgure below.
Here, the base is tied to ground via capacitor, and the signal is fed
to the emitter of Q1 through a capacitor. This circuit will perform
very similarly to the figure above, but will have slightly highly gain
reaching perhaps 500 and about the same low noise performance.
Ten microfarad capcitors are used thoughout because they are small ad
cheap, and are more than enough to do the job here.
This simple circuit can be made up in a ball smaller than an acorn
and put into mikes to give you more gain than you need to drive even
the worst transmitter. It also works well when driven by a speaker put
out in the yard to let you listen for prowlers at night, when you don't
have to get out of a warm bed, but the dog barks like he's on to
something. Fed into any hi-fi input, such a preamp will let you hear
better than if you were out in the yard. Here are many other uses, and
most of them will please you because the low noise of this preamp lets
you really hear clean audio.
┌────────┬────────── + 6 to 18 V
General Purpose / /
Low Impedence Preamp \ 220K \ 6.8K
/ Q2 / 10v 10µF
Q1 │ ┌─C─┴────────┤(────── Output
┌─C──┴───B┤2N3904 +
┌───────────────B┤2N3904 └─E>┐
│ └─E>┐ │
│ │ 100K │
├─────────────────── │ ──\/\/──┤
10 µF │ │ │
Input ─────)├────────────────┐ │ │
Z + +─┴─ C2 │ │ │
4Ω to 500Ω ─┬─10µF │ / /
│ │ \ 1K \ 1K
│ │ / /
│ │ │ │
- ────────────┴──────────┴─────────┴─────────┴──────────────── -