25-07-2012, 11:24 AM
A Single-Supply Op-Amp Circuit Collection
A Single-Supply Op-Amp.pdf (Size: 163.12 KB / Downloads: 18)
Introduction
There have been many excellent collections of op-amp circuits in the past, but all of them focus
exclusively on split-supply circuits. Many times, the designer who has to operate a circuit from a
single supply does not know how to do the conversion.
Single-supply operation requires a little more care than split-supply circuits. The designer should
read and understand this introductory material.
Split Supply vs Single Supply
All op amps have two power pins. In most cases, they are labeled VCC+ and VCC-, but sometimes
they are labeled VCC and GND. This is an attempt on the part of the data sheet author to
categorize the part as a split-supply or single-supply part. However, it does not mean that the op
amp has to be operated that way— it may or may not be able to operate from different voltage
rails. Consult the data sheet for the op amp, especially the absolute maximum ratings and
voltage-swing specifications, before operating at anything other than the recommended
power-supply voltage(s).
Most analog designers know how to use op amps with a split power supply. As shown in the left
half of Figure 1, a split power supply consists of a positive supply and an equal and opposite
negative supply. The most common values are ±15 V, but ±12 V and ±5 V are also used. The
input and output voltages are referenced to ground, and swing both positive and negative to a
limit of VOM±, the maximum peak-output voltage swing.
A single-supply circuit (right side of Figure 1) connects the op-amp power pins to a positive
voltage and ground. The positive voltage is connected to VCC+, and ground is connected to VCCor
GND. A virtual ground, halfway between the positive supply voltage and ground, is the
reference for the input and output voltages. The voltage swings above and below this virtual
ground to the limit of VOM±. Some newer op amps have different high- and low-voltage rails,
which are specified in data sheets as VOH and VOL, respectively. It is important to note that there
are very few cases when the designer has the liberty to reference the input and output to the
virtual ground. In most cases, the input and output will be referenced to system ground, and the
designer must use decoupling capacitors to isolate the dc potential of the virtual ground from the
input and output (see section 1.3).
Virtual Ground
Single-supply operation requires the generation of a virtual ground, usually at a voltage equal to
Vcc/2. The circuit in Figure 2 can be used to generate Vcc/2, but its performance deteriorates at
low frequencies.
AC-Coupling
A virtual ground is at a dc level above system ground; in effect, a small, local-ground system has
been created within the op-amp stage. However, there is a potential problem: the input source
and output load are probably referenced to system ground, and if the op-amp stage is connected
to a source that is referenced to ground instead of virtual ground, there will be an unacceptable
dc offset. If this happens, the op amp becomes unable to operate on the input signal, because it
must then process signals at and below its input and output rails.
The solution is to ac-couple the signals to and from the op-amp stage. In this way, the input and
output devices can be referenced to ground, and the op-amp circuitry can be referenced to a
virtual ground.
Combining Op-Amp Stages
Combining op-amp stages to save money and board space is possible in some cases, but it
often leads to unavoidable interactions between filter response characteristics, offset voltages,
noise, and other circuit characteristics. The designer should always begin by prototyping
separate gain, offset, and filter stages, then combine them if possible after each individual circuit
function has been verified. Unless otherwise specified, filter circuits included in this document
are unity gain.
Selecting Resistor and Capacitor Values
The designer who is new to analog design often wonders how to select component values.
Should resistors be in the 1-W decade or the 1-MW decade? Resistor values in the 1-kW to
100-kW range are good for general-purpose applications. High-speed applications usually use
resistors in the 100-W to 1-kW decade, and they consume more power. Portable applications
usually use resistors in the 1-MW or even 10-MW decade, and they are more prone to noise.
Basic formulas for selecting resistor and capacitor values for tuned circuits are given in the
various figures. For filter applications, resistors should be chosen from 1% E-96 values (see
Appendix A). Once the resistor decade range has been selected, choose standard E-12 value
capacitors. Some tuned circuits may require E-24 values, but they should be avoided where
possible. Capacitors with only 5% tolerance should be avoided in critical tuned circuits— use 1%
instead.