LM7171: Overheating problem in inverting amplifier configuration

Hi Conor,

the LM7171 is a 200MHz HF-OPAmp and your feedback resistances are way too high! With such high feedback resistances the OPAmp can become instable and might start to oscillate. This could be the cause for the heating up.

The LM7171 is designed for feedback resistances in the low kOhm range. See section 7.6 of datasheet.

Also, do not directly touch the output of LM7171 with the scope probe. Insert a 50R isolation resistor as discussed in section 8.3 of datasheet.

Kai

Hi Kai,

Thanks for the suggestions. I lowered the input resistance down to 100 Ohms and the feedback resistor to 1 kOhm. The probe I'm using has connecting wires to BNC so there is already a 50 Ohm impedance on the scope probe ideally. When I powered on the amplifier with the +- 15 V supply it overheated again. Is the supply current a possible issue with this? So I try powering the amp with +- 5 V first?

All the best,

Conor

Hi Conor,

it's a good idea to reduce the supply voltage to reduce the heat dissipation.

Can you tell more about your circuit? Is C really a 0.6pF cap?

Kai

Hi Kai,

The capacitor in the circuit is two metal 25 mm x 25 mm plates with a thin polymer dielectric between them. I calculated the capacitance to be around 0.6 pF using the dimensions and thickness of the dielectric. For certain the capacitance should be on the order of picofarads although there could be an error on this. The AC signal of 200 mV is being passed through these metal capacitors then onto the inverting amplifier setup as I showed in my diagram. I'm intend to get this circuit to measure solar cell transients capacitavely and the AC/square wave signal is being used as a model for this.

Hope this clarifies the circuit and my application.

Conor

Hi Conor,

wouldn't it be better to use a non-inverting amplifier topology four your application? And do you really need to use a 200MHz HF-OPAmp? Would a slower OPAmp also do?

Kai

Hi Kai,

I could try a non inverting topology however if I'm not mistaken they tend to be less stable than inverting amplifier topologies.

A higher frequency op am is ideal since it would reduce the ringing in the square wave signals I will be measuring and provide the largest bandwidth. This circuit worked for the lower frequency OP07E op amp rather well, the idea was to swap this op amp for the LM7171 to improve the high frequency performance. If you have any suggestions on an op amp similar to the OP07E with a higher frequency bandwidth (>2 MHz) I'd gladly appreciate it.

All the best,

Conor