Non investing amplifier input impedance calculation
Equation 2 also tells us that we can use a small signal variation method to determine Rout. Figure 3 An ideal Op Amp can be represented as a dependent source as in Figure 3. The dependent source is Ao vd, where Ao is the Op Amp open-loop gain and vd is the differential input voltage. The input differential resistance, between the Op Amp inputs, is considered high, so I removed it for simplicity. The same with the common mode input resistances, between the non-inverting input and ground and the inverting input and ground.
The non-inverting input is connected to ground, because a fixed value voltage source does not bring any change from a small-signal variation point of view. Thus, we are in line with the general rule that the output resistance of a circuit is calculated with the circuit inputs connected to ground.
Inspecting the loop made by Ao vd, Ro, and RL, vout can be expressed as in the following equation. 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.
Where very high input impedance levels are required, FET input op-amps may be used. 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.
Input impedance elements for an op amp 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.
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. 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.
Basic inverting operational amplifier circuit The basic inverting amp circuit is shown above.
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The signal which is needed to be amplified using the op-amp is feed into the positive or Non-inverting pin of the op-amp circuit, whereas a Voltage divider using two resistors R1 and R2 provide the small part of the output to the inverting pin of the op-amp circuit. These two resistors are providing required feedback to the op-amp. In an ideal condition, the input pin of the op-amp will provide high input impedance and the output pin will be in low output impedance.
The amplification is dependent on those two feedback resistors R1 and R2 connected as the voltage divider configuration. Due to this, and as the Vout is dependent on the feedback network, we can calculate the closed loop voltage gain as below. Also, the gain will be positive and it cannot be in negative form.
The gain is directly dependent on the ratio of Rf and R1. Now, Interesting thing is, if we put the value of feedback resistor or Rf as 0, the gain will be 1 or unity. And if the R1 becomes 0, then the gain will be infinity. But it is only possible theoretically. In reality, it is widely dependent on the op-amp behavior and open-loop gain. Op-amp can also be used two add voltage input voltage as summing amplifier. Practical Example of Non-inverting Amplifier We will design a non-inverting op-amp circuit which will produce 3x voltage gain at the output comparing the input voltage.
We will make a 2V input in the op-amp. We will configure the op-amp in noninverting configuration with 3x gain capabilities. We selected the R1 resistor value as 1. R2 is the feedback resistor and the amplified output will be 3 times than the input. Voltage Follower or Unity Gain Amplifier As discussed before, if we make Rf or R2 as 0, that means there is no resistance in R2, and Resistor R1 is equal to infinity then the gain of the amplifier will be 1 or it will achieve the unity gain.
As there is no resistance in R2, the output is shorted with the negative or inverted input of the op-amp. As the gain is 1 or unity, this configuration is called as unity gain amplifier configuration or voltage follower or buffer. As we put the input signal across the positive input of the op-amp and the output signal is in phase with the input signal with a 1x gain, we get the same signal across amplifier output. Thus the output voltage is the same as the input voltage.
So, it will follow the input voltage and produce the same replica signal across its output. This is why it is called a voltage follower circuit. The input impedance of the op-amp is very high when a voltage follower or unity gain configuration is used. Sometimes the input impedance is much higher than 1 Megohm. Moreover, for an ideal circuit, Ri is supposed to be infinite, as a consequence, no currents can enter the op-amp through any input because of the presence of an open circuit.
This observation can also be summarized by saying that the node interconnecting the inverting input and resistances R1 and R2 is a virtual short. For this same reason, all the feedback current across R1 I is also found across R2. Instead, the input impedance has a high but finite value, the output impedance has a low but non-zero value.
The non-inverting configuration still remains the same as the one presented in Figure 1. As a consequence, Equation 4 is simplified back to Equation 2. Even if for real op-amps, a small leaking current enters the inverting input, it is several orders of magnitude smaller than the feedback current.