OPA2336: GBWP of OPA2336

Shibin Varghese

Part Number: OPA2336
Other Parts Discussed in Thread: OPA336, OPA391, OPA392

Hi Team,
I want to measure the current through a resistor that is connected to the low side.

The Resistor R13 (2E) is the sense resistor.

The H Bridge is switching at 22KHz.
The expected current through the sense resistor is 100mA.
So the voltage developed across the resistor will be 100mA*2E=200mV.
I need to amplify this 200mV to about a 3V range, so I need a gain of approximately 10.
The feedback resistor of the op-amp OPA2336 is taken as 100K and the gain resistor is taken as 10.5K.
The Gain-Bandwidth Product of OPA2336 is only100KHz.
The H bridge is switching at 23kHz, since the current sensing is outside the H-bridge, the current signal is rectified to 46kHz.
Maximum amplification will be 100KHz/46KHz = 2.17
My required amplification is 11.
Is my calculation correct?. Please correct me If I am wrong.
Could you please explain the relationship between the GBWP and the maximum input frequency of an op-amp?.

over 1 year ago

0 Marek Lis over 1 year ago

TI__Guru 60470 points

Shibin,

The small signal bandwidth is a function of a close-loop Gain: BW=GBW/Gain. Thus, in Gain=10, OPA336 small-signal bandwidth is just 10kHz (100kHz/10) - please review TI precision lab training material under following link: https://training.ti.com/ti-precision-labs-op-amps-bandwidth-bode-plots-cutoff-frequency?context=1139747-1139745-14685-1138800-13124

However, even greater limitation of your application comes from OPA336 slew rate of 0.03V/us, which further limits the large signal to just 0.5V at 23kHz - this is called a full-power bandwidth and it is shown in the graph below.

Therefore, OPA336 is much too slow for what you need - see below.

Thus, you need an op amp with a higher slew rate like OPA391 (SR=1V/us) or even better OPA392 (SR=4V/us) - see below.

Having said that, the time-constant (C19||R13) is around 10us, thus in order to allow reading of 23kHz signal, you need to lower the value of C19 cap by at least 10x - see below.