"Quick and Dirty" Audio Amplifiers

For those of you who like to experiment with audio circuits and would like a "quick and dirty" amplifier that frees you from having to figure out the biasing resistors, we have two for you (and they run off 9 Volts too!). One uses an Op-Amp (Figure 1) and the other uses a transistor (Figure 2).

Both circuits need capacitors on the input and output to block DC while passing AC. The capacitor values depend on which circuit you use and what signal frequency you are amplifying. Start with a 1uF and go from there. Low frequencies may require a bigger value. (See Figure 1)

While Op-Amps normally run off of a dual voltage supply (+V and -V), it is possible to run them from a single voltage by using two equal value resistors (Rb) to create a separate DC grounding point midway between Vcc and actual ground, just for biasing the Op-Amp. The DC ground is connected to actual ground through a by-pass capacitor. The value of Rb is not critical; 10K should work just fine. To minimize DC offset in the output, Rb should have a 1% tolerance.

The gain of the amplifier is set by R1 and R2 {Av = R2/R1}. R2 should be 2K or bigger so as not to load the Op-Amp too much. If you use a bipolar device such as the venerable 741, the output can't go lower than 2 volts above ground or higher than 2 volts below Vcc. So with a 9-volt battery, the maximum output swing will be 5 volts: from 2V to 7V. If you want to go "rail-to-rail" from Vcc to Ground, then use a CMOS device like the CA3130; Vcc can then be as high as the Op-Amp allows. The CA3130 requires a 100 pF compensation capacitor.

Fig. 2 Schematic

If you really want "quick and dirty", this one transistor circuit is an 'oldie but goodie'. (See Figure 2). Note that by connecting the base-bias resistor Rb to the collector you get two benefits: 1) the biasing can not cause saturation or cut-off and 2) you introduce some negative feedback into the signal path which reduces distortion. It's not as good as the Op-Amp circuit but it does work. As for gain, you'll just have to measure it and see. Play around with different values of Rc, and make Rb = 100Rc. The input signal should not exceed 30mVp-p.

Parts List (Based on an AF signal of 1khz):

For Circuit #1         Circuit #1 w/CA3130:        For Circuit #2:
----------------------------------------------------------------------
1 LM741 Op Amp         1 CA3130 Op Amp             1 2N3904 Transistor
2 1 uF   Capacitor     2 1uF Capacitor             2 1 uF Capacitor
1 10 uF Capacitor      1 10 uF Capacitor           1 1K .5W Resistor
4 10K .5W 1% Res       1 100pF Capacitor           1 10K .5W Resistor
1 100K .5W Res         4 10K .5W 1% Res            1 100K .5W Resistor
1 9V Battery           1 100K .5W Res              1 9V Battery
1 9V Battery Snap                                  1 9V Battery Snap

	Part No. Description Price
	3200AUDAMP1 Circuit #1 $1.50
	3200AUD3130 Circuit #1 w/CA3130 1.75
	3200AUDAMP2 Circuit #2 1.10

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