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Operational amplifier circuit input impedance is important for a variety of reasons. It determines the loading on the previous stage, and also the impedance along with the lowest frequencies required determines the value of any coupling capacitor needed.
In view of this. It is necessary to understand the input impedance of the operational amplifier circuit, so that the required electronic circuit design decisions can be made.
The overall input impedance is not just the input DC resistance, but it is also complicated by the level of capacitance and this can have a marked effect on the overall impedance. This means that the effective circuit contains not only contains resistors but also capacitors.
Op amp input impedance basics
When referring to the op amp input impedance it is necessary to state whether it is the basic chip itself or the circuit:
Op amp chip input impedance: The input impedance of the basic integrated circuit is just the input impedance of the basic circuitry inside the chip. Some current is required to drive the base junctions of the input transistors, and this is one reason why the input impedance is not infinite.
In addition to this there is capacitance arising from the junction capacitance levels as well as the capacitance between the leads. This capacitance can be represented as distinct capacitors in an equivalent circuit.
Op amp circuit input impedance: Placing circuitry around an operational amplifier alters its input impedance considerably. Both the external electronic components and the way in which the feedback is applied affect the impedance.
This means that dependent upon the way in which the feedback is applied and the components used can vary in overall circuit input impedance from low values up to very high values.
As with any circuit there will be some capacitance as well. The effect of any inductance within the circuit is minimal in view of the frequencies generally used with operational amplifiers and this can be ignored.
The level of input impedance for the basic chip can be obtained from the operational amplifier data sheets where the input impedance is quoted, often in terms of MΩ. Where very high input impedance levels are required, FET input op-amps may be used.
When looking at the input impedance of FET input operational amplifiers, the input impedance of the op amp itself can be several TeraΩ This means that any loading effect due to the resistance only is exceedingly high, although capacitance effects can reduce the overall impedance quite significantly.
When looking at the integrated circuit data sheets, it is sometimes seen that the op amp input impedance is stated for differential and common-mode input cases. Typically current feedback op amps normally specify the impedance to ground at each input.
From this it can be seen that there are three resistors giving rise to chip input impedance. While for most cases the op amp resistance will be seen, at higher frequencies this may become slightly reactive and is more correctly termed an impedance. Typically the input resistance is of the order of 100 kΩ to 100 MΩ or more. The shunt capacitance may only be a few picofarads, often around 20pF or so
Although the basic resistance may be very high, even small levels of capacitance can reduce the overall impedance, especially as frequencies rise. The impedance of a 20pF capacitor is only 80kΩ at 100 kHz, or 800kΩ at 10 kHz.
This can mean that the overall impedance is dominated by the capacitive effect as frequencies rise.
Effect of feedback on input impedance
The circuit configuration and the level of feedback also have a major impact upon the input impedance of the whole op-amp circuit. It is not just the impedance of the amplifier chip itself - the electronic components around it have a significant effect.
The feedback has different effects, lowering or increasing the overall circuit impedance or resistance dependent upon the way it is applied.
The two main examples of feedback changing the input impedance or input resistance of an op-amp circuit are the inverting and no-inverting op-amp circuits.
Inverting op-amp circuit input impedance
The inverting amplifier using op-amp chips is a very easy form of amplifier to use. Requiring very few electronic components - in fact it is just two resistors, this electronic circuit provides an easy amplifier circuit to produce.
The basic inverting amp circuit is shown above. In order that the circuit can operate correctly, the difference between the inverting and non-inverting inputs must be very small - the gain of the chip is very high and therefore for a small output voltage, the difference between the two inputs is small. This means that inverting input must be at virtually the same potential as the non-inverting one, i.e. at ground.
As a result the input impedance of this op amp circuit is equal to the resistor R1. Normally this is relatively low and may be of the order of 1 kΩ or thereabouts dependent upon the actual electronic component values chosen. However this circuit does have the advantage of the virtual earth point at the inverting input of the op amp IC itself and this can enable it to be used as a virtual earth mixer.
Non-inverting op-amp circuit input impedance
The non-inverting amplifier offers the opportunity of providing a very high input impedance level.
Like the inverting amplifier, this one also uses very few electronic components. Again the basic form of the circuit uses just two resistors. The signal is applied to the non-inverting input and the feedback has a resistor from the output tot he inverting input, and another resistor from the inverting input to ground.
it is found that the input impedance for this op amp circuit is at least the that between non-inverting and inverting inputs, which is typically 1 MΩ to 10 TΩ, plus the impedance of the path from the inverting input to ground i.e. R1 in parallel with the resistor R2. it is this circuit that is used when requiring very high levels of input impedance.
Operational amplifier input impedance is a key issue for the design of any overall electronic circuit using op amps. The input impedance needs to be sufficiently high not to degrade the performance of the previous stages.
Accordingly there is a balance between the advantages of the inverting amplifier with its virtual earth mixing capability and simplicity, but low input impedance against the much high input impedance of the non-inverting amplifier.
Often the choice is down to individual preference, but either way the input impedance must be taken into account, whether high or low.
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