Electronics Components: Closed Loop Amplifiers

Open loop op-amp circuits aren’t very useful as electronic amplifiers because they’re so easily saturated. To make an op amp useful as an amplifier, you must use it in a feedback circuit, which reduces the gain to a more manageable amount so that input voltages that are usable (and even measurable!) can be amplified reliably.

You’re likely already familiar with the concept of feedback. You’ve probably sat in an auditorium listening to someone talk into a public-address system when suddenly a piercing screech came out of the speakers. That screech was feedback. In the case of the public-address system, the microphone picked up some of the output from the speakers and sent it back through the amplifier again. The result was an annoying high-pitched squeal.

Not all feedback is bad, though. In an op-amp amplifier circuit, feedback is used to reduce the enormous open loop amplification gain to a more manageable gain, such as 10. To do this, the output signal is fed back into the input via the V+ terminal. A resistor is used to reduce the voltage that is fed back to the input.

This type of circuit is called a closed loop amplifier because a closed circuit path exists between the output and the input. (Now you understand why an op-amp circuit without the feedback loop is called an open loop amplifier.)

The most common op-amp configuration is called an inverting amplifier because the voltage of the output is opposite the voltage of the input.

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In an inverting amplifier circuit, the input signal works its way through a resistor on its way to the V input, and the output is looped back into the V– input through a second resistor. These resistors are designated R1 and R2. You can easily calculate the overall voltage gain of the circuit by using this formula:

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Here, the gain is designated ACL (CL stands for closed loop).

If R1 is 1 k and R2 is 10 k, the voltage gain of the circuit will be –10. Then if the input voltage is +0.5 V, the output voltage will be –5 V (0.5 –10).

Note that the negative sign is required in an inverting amplifier circuit, so positive inputs give negative outputs, and vice versa.

A closed loop amplifier can also be designed as a noninverting amplifier in which the output voltage is not reversed. To do that, you simply reverse the inputs. Instead of connecting the input voltage to V through a resistor and grounding V+, you ground V through a resistor and connect the input voltage to V+. The feedback circuit is the same; the output is connected to the V– input through resistor R2.

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The formula for calculating the gain for a noninverting amplifier is a little different from the formula for an inverting amplifier:

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If R1 is 1 k and R2 is 10 k, the gain is 11. Thus, an input voltage of +0.5 V will result in an output voltage of +5.5 V.

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