 #157 ML741/VoltageFollower

Test a voltage follower/buffer circuit with the ML741 discrete component opamp

This is a demonstration of a voltage follower circuit using the ML741 discrete component opamp. The general operation of a voltage follower is for the output to follow the non-inverting input, with a gain of 1, i.e.

``````Vout = Vin
`````` How it works

Fundamentally, an op-amp strives to keep its inverting an non-inverting inputs equal by modulating the output.

When the non-inverting input rises above the inverting input, the output will rise to offset the differential. Since there is 100% feedback to the non-inverting input, the output will immediately change to match the inverting input.

Voltage followers, also known as unity gain buffers, are often used to isolate circuit sub-systems since they offer - at least in the idealised op-amp model - infinite impedence at the input and zero impedance at the output.

Construction  ML741 on the right, and a standard UA741CN on the left waiting to be put to the test.. ML741 v “real” 741 Test

Here are some results comparing the behaviour of a standard UA741CN chip with the ML741 (protoboard version).

Setup:

• power is 5V single rail, i.e. V- = GND
• non-inverting input is fed a sine wave 1Vpp with 2.5V DC offset (with a 1kΩ resistor in series for redundant protection)
• the function generator sine wave input replaces the manual 10kΩ pot in the schematic above

Scope connections

• CH1: non-inverting input
• CH2: output/inverting input

I’ve purposely driven the input signal such that it clips the lower output rail of both the ML741 and UA741CN, because things get interesting around the rails.

At 20kHz

• both tracking the input voltage quite well
• clean clipping at the lower output rail
• ML741 again proves its ability to drive lower than the standard UA741CN

ML741: UA741CN: At 80kHz

• both struggling to drop the output voltage fast enough
• the poorer ML741 response time has effectively increased its lower output limit so it is now higher than the UA741CN
• ML741 is the first to get into trouble (from 40kHz at least)

ML741: UA741CN: Measurements in action… 