GCSE    Op Amps    Questions 0 to 30   -->  View All  

Gain   =   1 + R_f / R₁   =   V_out / V_in         The input impedance is infinity or equal to R_in if this is present.        The output impedance is low.

Two Ways to Draw the Same Circuit

• Voltage Gain = 1 + Rf / R1
• The input resistance is equal to that of the op-amp. (Infinity for an ideal op-amp).
• The dotted resistor might be needed to hold the non inverting input, DC potential, at zero volts. This resistor can be large so it has little effect on the circuit input resistance (1 to 10MΩ).
• The non-inverting amplifier has a very high input resistance (impedance). This makes it ideal for amplifying small signal voltages from devices that can not provide any significant current. Such devices include capacitor microphones and piezoelectric sensors.

Gain Measurement

• Use 22K for resistor "a".
• Use 10K for resistor "b".
• Measure V in and V out
• Plot a graph with V out on the Y axis and V in on the X axis.
• The gain is the gradient of the graph.
• Watch out for limiting when the output gets close to the power supply voltage.

Parasitic Oscillation

Parasitic oscillations are caused by unwanted positive feedback, usually at higher frequencies. Lack of decoupling, poor layout and stray capacitance and resistance on prototype boards are all possible causes. This circuit shows some techniques to minimise the risk of oscillation. The feedback capacitor restricts high frequency performance. Some experimentation might be needed to get the right value. In some cases, the circuit will never work if built onto prototype board. A soldered circuit with all component lead lengths minimised might be needed.