Operational Amplifiers
What is the output voltage given by from an op-amp?
A is the open loop voltage gain, i.e. when there is no feedback.
What is an operational amplifier and what does it often require?
An operational amplifier (op-amp) is a voltage amplifier which amplifies the difference between the voltages on its two input terminals. Op-amps are often require a dual balanced d.c. power supply, e.g. ± 15V.
What is the graph of open loop voltage gain against frequency for an op-amp?
As can be seen from the graph, the voltage gain decreases with frequency so that at a frequency of 1MHz, the voltage gain has fallen to 1. voltage gain x bandwidth = constant
What is feedback?
Feedback occurs when a portion of the output is allowed back into the input. This can occur in two ways; it can cancel out part of the input signal and so is called Negative Feedback, or it can reinforce the input signal and so is called Positive Feedback.
What do most op-amps have?
Most op-amps have very large voltage gains, often as high as 10^6. Such large voltage gains require considerable care to be taken if the circuit is to be stable and not prone to oscillation. It is essential for the output terminals to be well separated from the input terminals to prevent positive feedback occurring through stray capacitance.
What does the + and - terminal on an op-amp stand for?
The '+' input terminal is known as the non-inverting input and the '-' input terminal is known as the inverting input terminal.
If a op-amp is assumed to behave ideally what happens?
The open loop gain is very large (in practice it is only very large at low frequencies), The maximum output voltage is equal to the power supply voltage, (in practice it is about 2V less), It has infinite input impedance so no current passes into the input terminals, (in practice the input impedance is not infinite so there is a current of a few nano-amps), The output impedance is zero so it can supply any required current, (in practice the op-amp is designed to limit the current to a few milliamps), The output voltage is zero when the two inputs are equal, (in practice there is a small offset voltage which needs a variable resistor to balance out).
What is positive feedback?
This is when a proportion of the output signal is 'fed-back' to the input with a phase shift of 0°, i.e. in phase with the input signal. This has the effect of increasing the input signal and so increasing the apparent gain of the whole circuit. All oscillator and astable circuits rely on positive feedback to make them unstable and oscillate. When constructing amplifiers with a large voltage gain, especially radio frequency amplifiers, it is imperative to ensure that the output circuit is kept as far away as possible from the input circuit. Failure to do so will almost certainly lead the amplifier system to be unstable at certain frequencies or in the worst case, oscillate very strongly at a particular frequency and be completely uncontrollable. A good indication of the stability of an amplifier is to measure its voltage gain for a range of frequencies. Any un-explainable increase in voltage gain at a particular frequency is a good indication that the system is not as stable as it should be.
What is negative feedback?
This is when a proportion of the output signal is 'fed-back' to the input with a phase shift of 180°. This has the effect of cancelling out part of the input signal and so reduces the apparent gain of the whole circuit. Negative feedback is used in the inverting and non-inverting op-amp amplifier circuits in the next section in order to reduce the voltage gain of the circuit to the required amount. In doing so, it also has the advantage of increasing the bandwidth of the circuit and reducing any distortion that may be introduced by the amplifier.
How is a op-amp kept stable?
To help ensure that an op-amp is stable a small capacitor is connected between the output terminal and the inverting input terminal. The effect of this is to make the voltage gain of the op-amp decrease with increasing frequency.
