Operational amplifier, which has two input pins and one output pin. Two of the input pins, one is a positive phase input and one is a negative phase input. It is because there are positive phase input and negative phase input of operational amplifier that engineers develop proportional amplification circuit with operational amplifier, there are two types of positive phase proportional amplification circuit and negative phase proportional amplification circuit.
What is positive phase? And what is negative phase?
In terms of proportional amplification circuits, positive phase means that the output voltage of the op-amp is proportional to the input voltage; similarly, negative phase means that the output voltage of the op-amp is also proportional to the input voltage, but with an additional negative sign. For example
Positive phase proportional amplifier circuit
Why is this proportional amplifier circuit called a positive-phase proportional amplifier circuit? We don’t need to worry about it for now. We only know that one of the basic circuit characteristics of operational amplifiers is the virtual short and the virtual break. The current flowing through the two input pins of the op-amp is 0. That is, the current flowing through resistor R1 is also 0. The current flowing through resistor R3 is also equal to the current flowing through resistor R2. So we have Vin = VA. And according to the property of virtual short, the voltages of the two input pins of the op-amp are the same, so VA = VB. And because Vin = VA, we have Vin = VB. For the output voltage Vout, it forms a circuit with the R2 resistor and R3 resistor. According to the Ohm’s law of the circuit, the current flowing through resistor R2 and resistor R3 is equal to
I = Vout / (R2 + R3)
This is the one hand; on the other hand, for resistor R2, the current flowing through it is also equal to the voltage across it divided by its resistance value, that is
I = VB / R2
What voltage is VB? It is both the voltage across resistor R2 and the voltage at the negative phase input pin of the operational amplifier, which is numerically equal to the voltage at the positive phase input pin of the operational amplifier.
I = VB / R2 = VA / R2 = Vin / R2
By putting these equations for the current together, we can calculate
Vin / R2 = Vout / (R2 + R3)
With this equation, we can find the relationship between the output voltage of the op-amp, Vout, and the input voltage, Vin
Vout = Vin * (R2 + R3) / R2
Obviously, the resistance of resistor R2 plus resistor R3 is greater than the resistance of resistor R2, which means that the value of (R2 + R3) / R2 is greater than 1. At this level, Vout is greater than Vin, and the output voltage of the operational amplifier exceeds the input voltage, so the circuit achieves the function of amplification. Moreover, this amplification is (R2 + R3) / R2, which is only related to the resistance value of these two resistors. I don’t know if you have found that the output Vout and the input voltage Vin, it is both in phase. If the input voltage is positive, the output voltage is also positive; if the input voltage is negative, the output voltage is also negative. This is the principle of positive phase proportional amplifier circuit.
Negative phase proportional amplifier circuit
According to the principle of operational amplifier, the same calculation procedure can be used to calculate this proportional amplifier circuit, which has the relationship between the output voltage Vout and the input voltage Vin
Vout = -Vin * R3 / R1
This equation has a negative sign, so we call it a negative-phase proportional amplifier circuit, where the output voltage has an inverse relationship with the input voltage. These are the two proportional amplification circuit schemes for operational amplifiers, one for positive-phase proportional amplification and one for negative-phase proportional amplification.
