I have designed an transimpedance amplifier for an ultrasound transducer as follows:

Ultrasound transducer feature: C=1nF, R=1.2K, working frequency between 2-3 MHz enter image description here

When the transducer is connected everything works nicely, as soon as the transducer is disconnected as follows:

enter image description here

The circuit starts to oscillate. I assume the reason is that OPA838 is not unity gain stable and when there is no capacitance on the inverting input, we can not provide the minimum noise gain for it.

My question is how I can adjust my circuit, even if there is no senor connected, the circuit remains stable, but I expect after I adjust the circuit, I still get the same bandwidths (7 MHz) and amplification (60 dB) as before. What change do you suggest? (I don not want to change OPA838)



1 Answer 1


You need to increase the (non-inverting) gain to >6 for stability. A 180 pF capacitor between ground and the inverting input should do that. As that is small compared to the capacitance of your sensor, its effect on bandwidth and noise will be modest, but not zero. There's no free lunch.

  • \$\begingroup\$ thank you very much for nice suggestion, I will test the idea and share the result with you. \$\endgroup\$
    – Andromeda
    Commented Jun 25 at 16:12

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