AN-581: Biasing and Decoupling Op Amps in Single Supply Applications

Single or Dual Supply?

Battery-powered op amp applications such as those found in automotive and marine equipment have only a single available power source. Other applications, such as computers, may operate from the ac power lines but still have only a single polarity power source, such as 5 V or 12 V dc. Therefore, it is often a practical necessity to power op amp circuits from a single polarity supply. But single supply operation does have its drawbacks: it requires additional passive components in each stage and, if not properly executed, can lead to serious instability problems.

Common Problems with Resistor Biasing

Single supply applications have inherent problems that are not usually found in dual supply op amp circuits. The fundamental problem is that an op amp is a dual supply device and so some type of biasing, using external components, must be used to center the op amp’s output voltage at midsupply. This allows the maximum input and output voltage swing for a given supply voltage.

In some low gain applications, where input signals are very small, the op amp’s output can be lifted above ground by only 2 V or 3 V. But in most cases, all clipping needs to be avoided and so the output needs to be centered around midsupply.

The circuit of Figure 1 shows a simple single supply biasing method. This noninverting, ac-coupled, amplifier circuit uses a resistor divider with two biasing resistors, RA and RB, to set the voltage on the noninverting equal to VS/2. As shown, the input signal, VIN, is capacitively coupled to the noninverting input terminal.

This simple circuit has some serious limitations. One is that the op amp’s power supply rejection is almost entirely gone, as any change in supply voltage will directly change the VS/2 biasing voltage set by the resistor divider. Power Supply Rejection (PSR) is a very important (and frequently overlooked) op amp characteristic.

Since a one volt change on the supply line causes a one-half volt change at the output of the divider, the circuit’s PSR is only 6 dB. So, the normally high power supply rejection provided by any modern op amp, which greatly reduces any ac signals (and power supply hum) from feeding into the op amp via its supply line, is now gone.

Even worse, instability often occurs in circuits where the op amp must supply large output currents into a load. Unless the power supply is well regulated (and well bypassed), large signal voltages will appear on the supply line. With the op amp’s noninverting input referenced directly off the supply line, these signals will be fed directly back into the op amp often initiating “motor boating” or other forms of instability.

While the use of extremely careful layout, multicapacitor power supply bypassing, star grounds, and a printed circuit board “power plane,” may provide circuit stability, it is far easier to reintroduce some reasonable amount of power supply rejection into the design.

Decoupling the Biasing Network from the Supply

The solution is to modify the circuit, as shown in Figure 2. The tap point on the voltage divider is now bypassed for ac signals by capacitor C2, restoring some ac PSR. Resistor RIN provides a dc return path for the VS/2 reference voltage and also sets the circuit’s (ac) input impedance.

Many published applications circuits show a 100 kΩ/100 kΩ voltage divider for RA and RB with a 0.1 µF or similar capacitance value for C2. However, the –3 dB bandwidth of this network is set by the parallel combination of RA and RB and Capacitor C2 and is equal to:

Motor boating or other forms of instability can still occur, as the circuit has essentially no power supply rejection for frequencies below 30 Hz. So any signals below 30 Hz that are present on the supply line, can very easily find their way back to the + input of the op amp.

A practical solution to this problem is to increase the value of capacitor C2. It needs to be large enough to effectively bypass the voltage divider at all frequencies within the circuit’s passband. A good rule of thumb is to set this pole at one-tenth the –3 dB input bandwidth, set by RIN/CIN and R1/C1.

Note that the dc circuit gain is unity. Even so, the op amp’s input bias currents need to be considered. The RA/RB voltage divider adds considerable resistance in series with the op amp’s positive input terminal, equal to the parallel combination of the two resistors. Maintaining the op amp’s output close to midsupply requires “balancing” this resistance by increasing the resistance in the minus input terminal by an equal amount. Current feedback op amps often have unequal input bias currents, which further complicates the design.

Therefore, designing a single supply op amp circuit design that considers input bias current errors as well as power supply rejection, gain, input and output circuit bandwidth, etc., can become quite involved. However, the design can be greatly simplified by using a “cookbook” approach. For a common voltage feedback op amp operating from a 15 V or 12 V single supply, a resistor divider using two 100 kΩ resistors is a reasonable compromise between supply current consumption and input bias current errors. For a 5 V supply, the resistors can be reduced to a lower value such as 42 kΩ. Finally, some applications need to operate from the new 3.3 V standard. For 3.3 V applications, it is essential that the op amp be a “rail-to-rail” device and be biased very close to midsupply; the biasing resistors can be further reduced to a value of around 27 kΩ.

Note that current feedback op amps are typically designed for high frequency use and a low-pass filter is formed by R2 and stray circuit capacitance, which can severely reduce the circuit’s 3 dB bandwidth. Therefore, current feedback op amps normally need to use a fairly low resistance value for R2. An op amp such as the AD811, which was designed for video speed applications, typically will have optimum performance using a 1 kΩ resistor for R2. Therefore, these types of applications need to use much smaller resistor values in the RA/RB voltage divider to minimize input bias current errors.

Instead of a bipolar device, the use of a modern FET input op amp will greatly reduce any input bias current errors 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.

Table 1 provides typical component values for the circuit of Figure 2 for several different gains and 3 dB bandwidths.

 Gain Input BW (Hz) Output BW (Hz) CIN* (μF) R1 (kΩ) C1* (μF) C2 (μF) COUT (μF) RLOAD (kΩ) 10 10 10 0.3 16.5 1.5 3 0.2 100 20 10 10 0.3 7.87 3 3 0.2 100 10 50 50 0.1 16.5 0.3 0.6 0.05 100 101 20 20 0.2 1.5 6.8 2 0.1 100 *Capacitance values rounded off to next highest common value. Since the CIN/RIN pole and C1/R1 poles are at the same frequency, and both affect the input BW, each capacitor is √2 larger than it would otherwise be for a single pole RC-coupled input. C2 is selected to provide a corner frequency of 1/10th that of the input BW.

Table 2 provides typical component values for several different gains and 3 dB bandwidths.

 Gain Input BW (Hz) Output BW (Hz) R1 (kΩ) C1* (μF) C2* (μF) COUT (μF) RLOAD (kΩ) 10 10 10 2 8.2 0.5 0.2 100 20 10 10 1 20 0.5 0.2 100 10 50 50 2 2 0.1 0.05 100 100 20 20 1 8.2 0.3 0.1 100 *Capacitance values rounded off to next highest common value. Since the C1/R1 pole and C2/RA/RB poles are at the same frequency, and both affect the input BW, each capacitor is √2 larger than it would otherwise be for a single pole RC-coupled input.

Figure 3 shows a circuit similar to Figure 2, but for an inverting amplifier.

Zener Diode Biasing

Although the resistor divider biasing technique is low cost, and always keeps the op amp’s output voltage at VS/2, the op amp’s common-mode rejection is entirely dependent upon the RC time constant formed by RA/RB and capacitor C2. Using a C2 value that provides at least 10 times the RC time constant of the input RC coupling network (R1/C1 and RIN/CIN) will help ensure a reasonable common-mode rejection ratio. With 100 kΩ resistors for RA and RB, practical values of C2 can be kept fairly small as long as the circuit bandwidth is not too low. However, another way to provide the necessary VS/2 biasing for single supply operation is to use a Zener diode regulator. Just such a scheme is shown in Figure 4. Here, current flows through resistor RZ to the Zener. Capacitor CN helps prevent any Zener-generated noise from feeding into the op amp. Low noise circuits may need to use a larger value for CN than the 10 µF specified.

A Zener should be chosen that has an operating voltage close to VS/2. Resistor RZ needs to be selected to provide a high enough Zener current to operate the Zener at its stable rated voltage and to keep the Zener output noise low. It is also important to minimize power consumption (and heating) and to prolong the life of the Zener. As the op amp’s input current is essentially zero, it’s a good idea to choose a low power Zener. A 250 mW device is best but the more common 500 mW types are also acceptable. The ideal Zener current varies with each manufacturer but practical IZ levels between 5 mA (250 mW Zener) and 5 µA (500 mW Zener) are usually a good compromise for this application.

Within the operating limits of the Zener, the circuit of Figure 4 basically restores the op amp’s power supply rejection. But this does not come without a price: the op amp’s output is now at the Zener voltage rather than at VS/2. If the power supply voltage drops, nonsymmetrical clipping can occur on large signals. Furthermore, the circuit now consumes more power. Finally, input bias currents still need to be considered. Resistors RIN and R2 should be close to the same value to prevent input bias currents from creating a large offset voltage error.

Figure 5 is an inverting amplifier circuit using the same Zener biasing method.

Table 3 can be used with circuits 4 and 5 to provide practical RZ resistor values for use with some common Zener diodes. Note that for the lowest possible circuit noise, the optimum Zener current should be selected by referring to the Zener product data sheet.

 Supply Voltage (V) Zener Voltage (V) Zener Type Zener Current IZ RZ Value (Ω) +15 7.5 1N4100 500 µA 15 k +15 7.5 1N4693 5 mA 1.5 k +12 6.2 1N4627 500 µA 11.5 k +12 6.2 1N4691 5 mA 1.15 k +9 4.3 1N4623 500 µA 9.31 k +9 4.3 1N4687 5 mA 931 +5 2.4 1N4617 500 µA 5.23 k +5 2.7 1N4682 5 mA 464

Tables 4 and 5 provide practical component values for Figures 4 and 5 for several different circuit gains and bandwidths.

 Gain Input BW (Hz) Output BW (Hz) CIN* (μF) R1 (kΩ) C1* (μF) COUT (μF) RLOAD (kΩ) 10 10 10 0.3 11.0 2 0.2 100 20 10 10 0.3 5.23 4.7 0.2 100 10 50 50 0.1 11.0 0.47 0.05 100 101 20 20 0.2 1.0 15 0.1 100 *Capacitance values rounded off to next highest common value. Since the CIN/RIN pole and C1/R1 poles are at the same frequency, and both affect the input BW, each capacitor is √2 larger than it would otherwise be for a single pole RC-coupled input.
 Gain Input BW (Hz) Output BW (Hz) R1 (kΩ) C1* (μF) C2* (μF) COUT (μF) RLOAD (kΩ) 10 10 10 10 2.7 0.2 0.2 100 20 10 10 5 4.7 0.2 0.2 100 10 50 50 10 0.5 0.05 0.05 100 100 20 20 1 12 0.1 0.1 100 *Capacitance values rounded off to next highest common value. Since the C1/R1 pole and C2/RIN poles are at the same frequency, and both affect the input BW, each capacitor is √2 larger than it would otherwise be for a single pole RC-coupled input.

OP Amp Biasing using a Linear Voltage Regulator

For op amp circuits operating from the new 3.3 V standard, a 1.65 V biasing voltage is needed. Zener diodes are commonly available only down to 2.4 V. The easiest way to provide this biasing voltage is to use a linear voltage regulator, such as the ADM663A or ADM666A devices. This is shown in Figure 6.

Although a Zener diode is usually the cheapest voltage regulator available, a linear voltage regulator has lower drift over temperature than a Zener and far less noise.

Resistors RA and RB are selected to provide the desired VS/2 voltage reference; consult AD663A datasheet.

DC-Coupled Battery-powered Circuits

So far, only ac-coupled op amp circuits have been discussed. Although with the use of suitably large input and output coupling capacitors, an ac-coupled circuit can operate at frequencies well below 1 Hz, some applications require a true dc response.

Battery-powered applications permit the use of a “phantom ground” circuit as shown in Figure 7. This provides dual supply voltages, both positive and negative with respect to ground, from a single battery. An op amp is used to buffer the output of a VS/2 voltage divider. If a low voltage battery such as 3.3 V is used, the op amp should be a “rail-to-rail” device and able to operate effectively from this supply voltage. The op amp also needs to be able to supply an output current large enough to power the load circuit. Capacitor C2 bypasses the voltage divider output enough to prevent any resistor noise from feeding into the op amp. This capacitor does not need to provide power supply rejection because the load current flows directly to ground and so any signal currents flow equally from both sides of the battery.

Resistors RA and RB are selected to provide the desired VS/2 voltage reference; consult AD663A datasheet.

Noise Issues

Some op amp applications need a low noise amplifier and low noise amplifier circuits require low resistance values in the signal path. Johnson (resistor) noise equals 4 nV times the square root of the resistance value in kΩ. While the Johnson noise of a 1 kΩ resistor is only 4 nV/√Hz, this increases to 18 nV/√Hz for a 20 kΩ resistor and 40 nV/√Hz for a 100 kΩ resistor. Even though the RA/RB resistor divider is bypassed to ground with a capacitor (C2), these resistors set a limit on the minimum value that can be used for the op amp’s feedback resistor and, the larger this is, the greater the Johnson noise. So low noise applications need to use much smaller op amp biasing resistor values than the 100 kΩ specified here. However, lower value resistors in the divider mean higher power supply current and reduced battery life.

Fortunately, the Zener diode biasing method supplies VS/2 without the need for large resistors. As long as the Zener is bypassed to keep its noise out of the circuit, both noise and supply current can be kept low. The use of a linear voltage regulator is even better, as its noise and output impedance are both very low.

Circuit Turn-on time Issues

One final issue that needs to be considered is circuit turn-on time. The approximate turn-on time will equal the RC time constant of the lowest BW filter being used.

The circuits shown here all call for the RA/RB, C2 voltage divider network to have a 10 times longer time constant than that of the input or output circuit. This is to simplify the circuit design (since up to three different RC poles set the input BW). This long time constant also helps keep the biasing network from “turning on” before the op amp’s input and output networks and, therefore, the op amp’s output gradually climbs from zero volts to VS/2 without “railing” to the positive supply line. The value supplied by this table is for a 3 dB corner frequency that is 1/10th that of R1/C1 and RLOAD/COUT. For example: in Figure 2, for a circuit BW of 10 Hz and a gain of 10, Table 1 recommends a C2 value of 3 µF, which provides a 3 dB bandwidth of 1 Hz.

Fifty thousand ohms (the parallel combination of RA and RB) times 3 microFarads equals an RC time constant of 0.15 seconds. So the op amp’s output will take 0.15 seconds (approximately) to settle to VS/2. The input and output RC networks will charge up ten times faster.

In some applications, where the circuit’s –3 dB low frequency bandwidth is very low, the circuit turn-on time may become excessively long. In that case, a Zener biasing method may be a better choice.