Electronics All-in-One For Dummies by Lowe Doug

Author:Lowe, Doug
Language: eng
Format: epub
Publisher: Wiley
Published: 2012-01-18T16:00:00+00:00

Figure 3-4: A op amp configured as an open loop amplifier.

In the open loop op-amp circuit, the V+ input is connected to ground, and an input signal is placed on the V– input. In this arrangement, the voltage to be amplified is the same as the voltage of the V– input. Although Figure 3-4 shows the input as alternating current, the open loop op-amp circuit works for direct current as well.

The voltage gain in an open loop op-amp circuit is extraordinarily high — on the order of tens or even hundreds of thousands. Suppose that you’re using an op amp whose open loop voltage gain is 200,000 and that the power supply is ±9 V. In that case, an input voltage of +0.000025 V will result in an output voltage of +5 V. An input voltage of +0.00004 V will give you an output voltage of 8 V.

The ideal op amp

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. So you see, in many ways, op amps are almost human.

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: Several times in this chapter, I mention that 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.

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