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Electronics Components: Open Loop Amplifiers

As its name suggests, one of the most basic electronic uses of an op amp is as an amplifier. If you connect an input source to one of the input terminals and ground the other input terminal, an amplified version of the input signal appears on the out terminal.

An important concept in op-amp circuits is voltage gain, which simply represents the amount by which the difference between the two input voltages is multiplied to produce the output voltage. If the input voltage difference is 2 V and the output voltage is 12 V, for example, the voltage gain of the amplifier is 6.

If you simply apply an input signal to the V–– terminal of an op amp, the circuit is called an open loop amplifier.

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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. The open loop op-amp circuit works for both alternating and direct current.

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 output voltage can never exceed the power supply voltage. In fact, the maximum output voltage usually is about 1 V less than the power supply voltage. So if you’re using a pair of 9 V batteries to provide a 9 V power supply, the maximum output voltage is 8 V.

As a result, the most that an open loop op-amp circuit with an open loop gain of 200,000 can reliably amplify is 0.00004 V. If the input voltage difference is any larger than 0.00004 V, the op amp is said to be saturated, and the output voltage will go to the maximum.

No matter how much money you invested in a top-quality voltmeter, it isn’t sensitive enough to measure voltages that small. Physicists at Cal Tech may be able to measure voltages that size, but for all practical purposes, 0.00004 V is the same as 0 V.

As a result, one of the basic features of an open loop op-amp circuit is that if the input voltage difference is anything other than zero, the op amp will be saturated, and the output voltage will be the same at its maximum. So if the maximum output voltage is 8 V, the output will be one of only three voltages: +8 V, 0 V, or –8 V.

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