# Single Supply AC Gain Opamp With Unity DC Gain

I'm trying to build a AC amplifier that will amplify an input signal of around 0.4Vptp @ 160kHz to around 1.7Vptp. I'm also attempting to do this with a single supply (3.3V) opamp. I built the circuit depicted below.

simulate this circuit – Schematic created using CircuitLab

After building and testing this circuit I ran into some very weird behavior.

1. The voltage at the non inverting is suppose to be around 1.65V but during measurement it was around 0.5V. My first assumption was possible the input bias current was causing this. After checking the datasheet the bias current is only 0.5pA and with a 2M resistor that should only cause a change of around 2uV.
2. Despite the weird bias I was able to get some scope measurements (note: yellow trace is at the non inverting pin of the opamp and the blue trace is at the output:

Over time the output started to clip and eventually the output became flat (over a periods of a few minutes) Once the output had flat lined I noticed the current being pulled by the opamp was around 50mA!!

I was hoping for some direction on this issue. I have no idea where to start.

A basic problem with attaching oscilloscope leads to high impedance nodes is that the input impedance of the o-scope circuits are loading those nodes. O-scopes typically have an input impedance of around 1 Mohm and, with an effective impedance of 2 Mohms || 2 Mohms your bias voltage of 1.65 volts is going to approximately halve to around 0.8 volts. If your o-scope probes have the option of a 10:1 operation, the effective impedance becomes more like 10 Mohms so, this would be the preference.

Another observation: you do need to have a supply bypass capacitor but your circuit doesn't show one. Data sheet extract: -

Connect low-ESR, 0.1-µF ceramic bypass capacitors between each supply pin and ground, placed as close to the device as possible. A single bypass capacitor from V+ to ground is adequate for single-supply applications

Next....

Once the output had flat lined I noticed the current being pulled by the opamp was around 50mA!!

This may be a fault on your circuit pulling down the 3.3 volt supply rail - please check to see at what point the output started to flat-line and confirm if it coincided with the draw of 50 mA or whether the 50 mA built up gradually over the period. As a side note, the TLV9062 is capable of outputting 50 mA into a short circuit so maybe there was an excessive load on the output?

You say you are using a TLV9062 but some of those parts come with a shutdown pin and it's not clear from your question whether you are using one of those parts and, if you are, how you are wiring the shutdown pin.

To ensure optimal shutdown behavior, the SHDN pins should be driven with valid logic signals. A valid logic low is defined as a voltage between V– and V– + 0.2 V. A valid logic high is defined as a voltage between V– + 1.2 V and V+. The shutdown pin must either be connected to a valid high or a low voltage or driven, and not left as an open circuit. There is no internal pull-up to enable the amplifier.

And, both shutdown pins need to be properly controlled to avoid problems even when only using one amplifier in the package. The other amplifier (if unused) should have applied sensible levels to its inputs as a precaution.

I was hoping for some direction on this issue. I have no idea where to start.

Some direction given!

• I'll give all these a try tomorrow and see what happens! Jan 13, 2020 at 9:14
• Turns out the bias resistors were changing values due to conductive flux!! This issue combined with the input impedance of the probe (1M) to cause this issue. Thank you for the input Jan 14, 2020 at 23:57