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Selection of High Speed OPamp

Other Parts Discussed in Thread: OP07, LF156, OPA140

I want to amplify a 48KHz sinusodial signal with a gain factor of 3 - 4 OP07 does not allow this. Iam using +/- 15V and input signal is @8Vp-p, output expected is 30Vp-p. But practically op07 does not support this than i used LF156 but the o/p does not go above 18Vp-p. After analysing i feel that the unity gain bandwidth for these opamps is very low. My question is can someone suggest the appriopriate opamp for this application. I am designing a capaccitance impedence converter circuit.


Ajay dudhe

  • Ajay,

    30Vp-p output on a 30V supply (+/-15) is a tall order for any opamp. Opamps with a "rail-to-rail" output can indeed swing their outputs very close to their supply rail, how "close" depends on the load impedance and how much linearity you are willing to sacrifice. For example, the OPA140 has a rail-to-rail output but looking at the datasheet we see in the specifications table:

    Notice that for a 10kOhm load, the OPA140 is able to get within 200mV of each rail, but for a 2kOhm load this number increases to 350mV. You should also see that open loop gain (Aol) is specified to be greater than or equal to 108dB for each case, which is an indication of how linear the opamp is operating. Overall, what you should take away from this explanation is that it is not reasonable to expect a 30Vpp undistorted signal from the output of an opamp running on +/-15V supplies.

    As for the small signal parameters, it is true that the unity gain bandwidth of your amplifier will need to be greater than 4 x 48 kHz (192kHz) however I think you will find that the slew rate of the amplifier is also a consideration for such a large output signal. For a 30Vp-p sine wave at 48kHz, the maximum rate of change of the waveform occurs at the zero crossing points and is defined by dv / dt = 2 * pi * f * A (where f is frequency and A is amplitude). Plugging in the values for your signal, you should see that the maximum rate of change for your signal is 4.52 Volts per microsecond. Here is where the OP07 falls short because it's slew rate is only .3 Volts per microsecond. The LF156 should be able to slew at 12 V/uS which is a better choice for you application, however the limited output swing you are seeing leads me to believe you are driving a very low impedance load.

    If you could clarify what the impedance is that the output of the opamp will see I may be able to recommend a more suitable part for your application.

  • Thanks for the reply.


    The output is connected to a load of 1K in parallel with 20pF capacitor, further it drives a differentiator circuit wherein capacitor is varying from 100pF to 200pF and feed back resistance of 51K. My aim is to measure the output voltage change due to this change in capacitance value.


    Awaiting for your reply.



  • Hi Ajay,

    It would really help me if you could draw a schematic of your circuit, I'm not sure I correctly understand how the differentiator is connected to the 1k//20pF load.



    Please find attached the circuit schematic. Kindly study and comment............


    Ajay dudhe