Soldering On (Part 6)
Operational amps as mixers
Tim Edwards cooks up another look at Op Amps with some basics of mixers
Continuing on the important subject of op amps, mention was made in the last issue that most op amps require split power supplies to operate successfully. Usually the supplies are balanced, i.e. equal positive and negative voltages are used, but this is not mandatory if the maximum positive and negative output swings are known to be unsymmetrical. The input signals are referenced with one side attached to 0V. If, however we require an amplifier with a single power supply then for AC signals this can be done simply. For DC coupled signals more care is needed and split supplies are usually the easiest and best solution.
Figure 1 shows an audio pre-amplifier using a single supply rail. The amplifier can be any common 741 type. R1 and R2 form a potential divider so that the non-inverting input is biased through R3 to half the supply voltage. C2 blocks the DC path to ground via R2 and so R4 provides the only current path to the inverting input at very low frequencies. Hence the amplifier has unity gain at DC and so the output is biased to the non inverting input voltage.This positive bias is not important because the capacitor C3 blocks the DC component of the signal before it is passed to the power amplifier. The audio signal will pass easily through C1, and C2 now also appears as a low impedance path to the AC component. The inverting input is connected to the ground via R1 and R2 (the supply rail appears the same as ground to alternating signals). So gain is now introduced at the audio frequencies and the value of the gain is given by 1 + R4/R5 where R5 is the value of R1 in parallel with R2. In this case the amplifiers will have a gain of about 20, assuming the frequencies are high enough such that the impedance of C2 can be ignored (150 ohms at 1000Hz).
Another very important configuration for use in audio circuits is the mixer circuit. Fig. 2 shows a basic mixer using split supplies.
The summing amplifier of mixer uses the virtual earth configuration with many inputs connected via R1, 2, 3 etc (usually identical in value). The inverting input is virtually at earth potential because the non-inverting input is tied to signal ground. Remember that the amplifier output acts to try to keep the voltage difference at the two inputs at a minimum. Since the inverting input is a virtual earth, then the input currents are equal to e1/R1, e2/R2, e3/R3 etc and must all pass through R4 as negligible current flows into the input. Therefore V out = (e1/R1 + e2/R2 + e3/R3) R4. ie the input voltages are summed together in the inverse ratio of the input resistors. The larger the input resistor the less that particular input will contribute to the output. In a mixing console these input resistors can be variable to give the desired degree of mixing or else a fixed value of input resistor (all the same value) and change the value of input voltage applied via a potentiometer.
Figure 3 shows a variation on the basic summing amplifier. Again the circuit uses a single power supply and so the input and output signals must be isolated via capacitors to remove the DC component. The input resistors prevent the signal from any one input line from reaching any other input.
Feature by Tim Edwards