Opamp DC Specification

DC Specification  of opamp

Vos, input offset voltage Output voltage caused by component mismatch, referenced to the input. This will be multiplied by the gain. For example, an opamp with a 2mV Vos connected as an inverter with a a gain of 1000 and the input grounded, will have the output sitting at -2v.

Normally synth circuits aren't as concerned with Vos because they tend to be used with low gains.

Ibias, Input bias current The inputs of opamps are either the base of a pnp/npn transistor, or the gate of a JFET/MOS transistor. Typically this is a few nanoAmps for bipolar inputs, or picoAmps for JFET/MOS types. The input bias current produces an apparent input voltage when flowing through input resistors.

In a synth circuit, use JFET or MOS input opamps for circuits that deal with very small currents - like integrators on VCO's (the input bias current will determine the low frequency), or Sample and Hold buffers.

Ios, input offset current Ios with matched input resistances causes the equivalent of a Vos. Typically the Ios is much smaller than the Ibias.

If you have input bias currents that exactly match, you can compensate for them by using matched resistance on the inputs. For example, on an inverter, have a resistance of Rf in parallel with Rin on the + input to ground (this is the equivalent resistance seen looking "out" the - input).

Don't bother doing this on JFET/MOS input opamps - the error is too small to make any difference.

CMRR, Common Mode Rejection Ratio This is a measure of an opamps' ability to act as a true differential amplifier. Ideally, if you apply the same signal to both inputs, you should get zero output. Such an opamp would have an infinite CMRR. CMRR is defined as the ratio of the change in common mode voltage to the change in output voltage, expressed in decibels. The spec is usually at dc, because it is very sensitive to frequency (it gets worse as frequency goes up). An opamp spec sheet will usually have a CMRR vs. Frequency graph.

Related to CMRR is the Common Mode Range, or how close to the supply rails an opamp can handle common mode input voltages. Typically it's within a couple of volts of the supplies. Common mode range can be important because some opamps do weird things when their common mode range is exceeeded. The TL07x series can change phase (+ input becomes - input, causing latchup). The OP27 can oscillate at 5MHz or so.

The voltage follower is the most common circuit where you will have to be concerned with common mode range.

PSRR, Power Supply Rejection Ratio This is a measure of an opamps' ability to ignore changes in the power supply voltages. Ideally, if you change the power supply voltages, you should no change in the output. Such an opamp would have an infinite PSRR. PSRR is defined as the ratio of the change in power supply voltage to the change in output voltage, expressed in decibels.

Since we normally do not change power supply voltages on purpose, the ripple on the supply lines becomes important. The frequency of this will be 120Hz on a full wave rectified linear supply. If you use a switching power supply, it is much much worse because the PSRR will be very low at the ultrasonic frequencies of the switching - the signal will get into everything. That's whay switching power supplies are not recommended for analog circuits.

Output swing This is a measure of how close to the power supply voltages the output can swing. Typical opamps can reach within 1v to 2v of the supplies. Special purpose "rail to rail" opamps can reach within a few millivolts of the supplies.

The output swing will determine what type of signal levels you can suppoprt in your synth circuits. For example, to support +/-10v (20vp-p) signals, you probably should use +/-15v supplies. If you are using +/-12v supplies, you probably want to standardize on +/-5v signal levels.

A side note: the LM324 quad opamp does not perform like a quad 741. It suffers from "crossover distortion". I've had control voltage circuits using an LM324 that didn't work right until I swapped it out for a TL074. I've heard that adding a resistor from the output to the negative supply (10K or so) improves this.

Keywords : Opamp, DC, Specification, Offset, Voltage, Current, CMRR, PSRR, Output Swing, Op-Amps, Amplifier
Writer : delon  |
19 Apr 2006 Wed   
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