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Opamp AC Specifications

AC Specification  of Operatioanl Amplifiers

GBW, Gain-Bandwidth Product Negative feedback differential amplifiers such as opamps depend on high open loop gains to support the feedback. The open loop gain of an opamp is high at dc, but decreases rapidly with frequency, usually at a one to one logarithmic rate i.e., Aol (open loop gain, not America OnLine) decreases by a factor of two every time the frequency doubles. The Gain Bandwidth Product is the frequency where the open loop gain has dropped to unity.

GBW gives a measure of how high a frequency the opamp can handle for small signals.

Slew Rate Slew Rate, usually expressed in volts per microsecond (V/uSec), is a measure of the large signal frequency handling capability of an opamp. It's actually a limitation of the amount of current available to charge capacitances internal to the opamp.

The graphs for large signal handling capability you see in opamp datasheets are based on the opamp's ability to output an undistorted sine wave. But in synths we have signals like saw and pulse waveforms that should have very straight up and down slopes - which requires an even higher slew rate.

For example, to handle large audio signals (say 10v p-p) requires a minimum slew rate of about 1.5V/uSec. This rules out the 741 types that only have 0.5V/uSec. It's why the JFET input opamps like the TL07x or TL08x series are so popular for synth circuits - they have slew rates of around 13V/uSec.

Noise All electronic components produce noise, and every noise source adds up and is amplified by whatever gains you have in your circuits. It's the hiss you hear when there is no signal present. The noise specs of an opamp become important when you are using a lot of gain to amplify small signals.

Signals in synth circuits tend to be fairly large, so using low noise opamps are not that important. The TL07x used so commonly in synth circuits actually have pretty high noise. You might want to use low noise opamps like the NE5532 or the OP27 types to amplify low level signals like microphones. Mixers are another good candidate.

Keywords : Opamp, AC, IC, Specification, Offset, Gain, Bandwidth, Slew Rate, Noise, Output Swing, Op Amps, IC, Amplifier, Circuit, Develop, Electronic, Integrated
Writer : delon  |
19 Apr 2006 Wed   
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