Although it is advantageous to implement op-amp circuits with balanced dual supplies, there are many practical applications where, for energy conservation or other reasons, single-supply operation is necessary or desirable. For example, battery power, in automotive and marine equipment, provides only a single polarity. The fundamental issue is that, if supply voltage ripple rejection signal is to swing both positive and negative with respect to “common”, this zero-signal reference voltage must be at a fixed level between the supply rails. The principal advantage of dual supplies is that their common connection provides a stable, low-impedance zero-reference.
Though apparently simple, there are problems with it. Illustrating the problem, the circuit of Figure 1, which has several design weaknesses, is an ac-coupled non-inverting amplifier. The signal is capacitively coupled in and out. A potentially unstable single-supply op-amp circuit. This simple circuit has additional potentially serious limitations. Even worse, instability can occur in circuits where the op-amp must supply large output currents into a load.
With the op-amp’s non-inverting input referenced directly off the supply line, these signals will be fed directly back into the op-amp, often in a phase relationship that will produce “motor boating” or other forms of oscillation. While the use of extremely careful layout, multi-capacitor power supply bypassing, star grounds, and a printed circuit board “power plane”, all help to reduce noise and maintain circuit stability, it is better to employ circuit design changes that will improve power supply rejection. The tap point on the voltage divider is now bypassed for ac signals by capacitor C2, to restore the ac power-supply rejection. A decoupled single-supply op-amp biasing circuit. A practical approach is to increase the value of capacitor C2. The amplifier’s gain at dc is still unity.
Even so, the op-amp’s input bias currents need to be considered. RB voltage divider, adds considerable resistance in series with the op-amp’s positive input terminal. Maintaining the op-amp’s output close to midsupply, using common voltage-feedback op amps that have symmetrical balanced inputs, can be achieved by balancing this resistance by the choice of R2. 12-V single supplies, down to 42 k ohms for a 5-V supply and 27 k ohms for 3. An op-amp such as the AD811, which was designed for video speed applications, typically will have optimum performance using a 1 k ohm resistor for R2. Because of their low bias current, the need for balancing input resistors is not as great in applications with modern FET-input op-amps, unless the circuit is required to operate over a very wide temperature range. In that case, balancing the resistance in the op-amp’s input terminals is still a wise precaution.
Figure 3 shows how biasing and bypassing might be applied in the case of an inverting amplifier. A decoupled single-supply inverting amplifier circuit. With 100-k ohm resistors for RA and RB, practical values of C2 can be kept fairly small as long as the circuit bandwidth is not too low. 2 biasing for single supply operation is to use a Zener-diode regulator, such as that shown in Figure 4. Here current is supplied to the Zener diode through resistor R. Capacitor CN helps reduce Zener-generated noise from appearing at the op-amp input.