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I am trying to use a non-inverting amplifier to boost a 1kHz signal with a DC component of 130mV and a peak to peak swing of 52mV.

I am using a uA741 General Purpose OPamp, powered via 5V from USB (VCC- = 0V, VCC+ = 5V). I have a 100kOhm resistor from Output to V- and a 10kOhm resistor from V- to GND. On paper, this should give me a gain of 11. The signal is being fed into V+. I have left the offsets unconnected for now. The whole system is implemented on a breadboard.

Here's a schematic of the circuit: uA741 in supposed 11x non-inverting feedback condition

However, when I apply 5V to this circuit the output is just glued to 4V. Interestingly, this is also the case when I connect V+ to GND.

I have removed all connections and reconnected everything to make sure that it's wired up correctly. I believe it is. I tried replacing the uA741 with an OPA27GP (just grabbing what's nearby) but with the same effect.

At this point I realize that I am out of my depth. I ran this circuit (with an ideal op-amp) on https://www.falstad.com/circuit/ and it performed just as I would expect.

Does anyone know what might be the issue here?

user2640555
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1 Answers1

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The uA741 does not function with a 5V supply. It requires at least +/-5V (10V between V+ and V-). The same applies to the OPA27, which is actually only specified to operate with +/-15V (derated down to +/-4V).

Use an OpAmp that's rated for operation at these voltages, for example the MCP6274. The OpAmp also needs to be a rail-to-rail type (which the MCP6274 is).

Jonathan S.
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  • Use a positive and negative supply. The inputs are at the same voltage as the negative power pin which will cause problems. The inputs want to be biased at mid-power supply level which is why a positive and negative power supply simplifies things. – qrk Feb 27 '24 at 00:52
  • @qrk, what could one do if the 5V USB rail was the only thing available? DC filter cap and a voltage divider on the other end to bring the signal up to mid-level? – user2640555 Feb 27 '24 at 06:18
  • @user2640555 A rail-to-rail OpAmp solves this. Your circuit will work without modification with a MCP6274, for example. – Jonathan S. Feb 27 '24 at 10:40