Electronics Components: The Ideal Op Amp - dummies

Electronics Components: The Ideal Op Amp

By Doug Lowe

If you read about op amps on the web or in an electronics book, you’ll undoubtedly come across the term ideal op amp. An ideal op amp is a hypothetical op amp with certain characteristics that real op amps strive to achieve. Real op amps come very close to the ideal op amp, but no op amp in existence actually achieves the perfection of an ideal op amp.

Depending on which list you read, an ideal op amp has anywhere between two and seven characteristics, the most important of which are

  • Infinite open loop gain: The open loop gain in an op amp is very large — on the order of tens or even hundreds of thousands. In an ideal op amp, the open loop gain is infinite, which means that any voltage differential on the two input terminals will result in an infinite voltage on the output.

    In real op amps, the output voltage is limited by the power supply voltage. Because the output voltage can’t be infinite, the gain can’t be infinite either.

  • Infinite input impedance: Impedance represents a circuit’s opposition to current flow, whether the current is alternating or direct. In an ideal op amp, the impedance of the two input terminals is infinite, which means that no current enters the op amp from the inputs. The inputs are able to see and react to the voltage, but that voltage is unable to push any current into the op amp.

    What that means in practice is that the op amp has no effect on the input voltage. In an actual op amp, a small amount of current — usually, a few milliamps or less — does leak into the op amp’s input circuits.

  • Zero output impedance: In an ideal op amp, the output circuitry has zero internal impedance, which means that the voltage provided from the output is the same regardless of the amount of load placed on it by the circuit to which the output is connected.

    In reality, most op amps have an output impedance of a few ohms, which means that the actual voltage provided by the output terminal will vary a small amount depending on the load connected to the output.

  • Zero offset voltage: The offset voltage is the amount of voltage at the output terminal when the two inputs are exactly the same. If you connect both inputs to ground, for example, there should be exactly 0 V at the output.

    In reality, real world op amps have a very small voltage on the output even when both inputs are grounded, connected to each other, or not connected to anything at all. For most op amps, this offset voltage is just a few millivolts.

  • Infinite bandwidth: The term bandwidth refers to the range of alternating current frequencies within which an op amp can accurately amplify. In an ideal op amp, the frequency of the input signal has no effect on how the op amp behaves. In real-world op amps, the op amp doesn’t perform well above a certain frequency — typically, a few megahertz (millions of cycles per second).

The characteristics are often summed up with the following two “golden rules” of op amps:

  1. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.

    This rule, which applies only to closed-loop amplifier circuits, means that the feedback sent from the output to the input causes the two input voltages to become the same.

  2. The input draws no current.

    This rule means that the input terminals look at the voltage placed across them but don’t allow any current to flow into the op amp.

Although no actual op amp is able to live up to the standards of the ideal op amp, most come pretty close. Close enough, in fact, that you can safely design an op amp circuit as if the op amps were ideal. In particular, the two golden rules apply: The feedback will equalize the input voltages, and the op amp draws no current from the input.