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OPA564: GBW Question of OPA564 vs THS3122, which is appropriate to excite/push a coil at ~150 kHz?

jQ
Prodigy 180 points
Part Number: OPA564
Other Parts Discussed in Thread: THS3122, BUF634

Hi,

I want to design an "AC excitation" module to excite a coil between 100 kHz and 200 kHz (R=2 Ohm at DC, Z=50 Ohm at 150 kHz).  I have a DDS (AD9833) which can generate sine wave with peak voltage of 0.65 V at 150 kHz.  I want to amplify the signal with a power amplifier which can amplify the voltage to around 12 V (Gain=18) and provide sufficient current.  I found two amplifiers: one is OPA564 and the other is THS3122.  THS3122 has 450 mA output current, 26 Vpp Output voltage (RL=50 Ω), and 128 MHz -3dB BW so it seems that the gain is flat above 150 kHz.

1. My first question is that whether OPA564 is also a choice?  My main concern is its GBW, which is 17 MHz.  Suppose the gain is 20, the BW is then 17M/20=850 kHz, does that mean it is still sufficient for this application (100 kHz-200kHz)? 

2. My second question is that if I want to excite the coil above 850 kHz (i.e. 1MHz), will OPA564 be a good choice still?  I guess the answer is no because at that frequency the gain is smaller than 20 and is not flat so the output voltage amplitude is not constant when the frequency is different.  Is it correct?

3. I also saw some people use a combination of op-amp and high-current buffer to achieve the function.  Does anyone have any suggestions of the selection of the components?

Thank you very much for your help!

  • 0
    TI__Guru* 93105 points

    Hi jQ,

    Please see my responses to your questions:

    1. My first question is that whether OPA564 is also a choice?  My main concern is its GBW, which is 17 MHz.  Suppose the gain is 20, the BW is then 17M/20=850 kHz, does that mean it is still sufficient for this application (100 kHz-200kHz)?

    There may be an issue up front that would prevent you from using the OPA564 in the application. The specified maximum operating voltage is 24 V, or +/-12 V. Since the application requires a peak output voltage of 12 Vpk, the supply voltage would need to be higher than +/-12 V; some output voltage headroom is needed to prevent clipping. The output swing is limited to about a volt or two from the supply rails at high current levels. Otherwise, viewing OPA564 datasheet graph 13, GAIN AND PHASE vs FREQUENCY, and graph 15, OUTPUT VOLTAGE SWING vs FREQUENCY, the OPA564 should have enough bandwidth and slew rate to support a 100  to 200 kHz large signal. The open-loop gain is only about 40 dB in that range and with a closed-loop gain of 18 V/V, there isn't much loop gain remaining. The gain error and distortion performance may not be as good as is needed.

    2. My second question is that if I want to excite the coil above 850 kHz (i.e. 1MHz), will OPA564 be a good choice still?  I guess the answer is no because at that frequency the gain is smaller than 20 and is not flat so the output voltage amplitude is not constant when the frequency is different.  Is it correct?

    If the OPA564 or any other fast power op amp is used to drive a coil the open-loop gain/phase and output impedance of the op amp must, in conjunction with the complex load impedance, be analyzed . Compensation may be required to achieve sufficient phase margin and assure stability.

    3. I also saw some people use a combination of op-amp and high-current buffer to achieve the function.  Does anyone have any suggestions of the selection of the components?

    Take a look at the BUF634 datasheet:

    The THS3122 appears to have specifications that meet your application's demands. The one concern is the power dissipation that it must handle under the application conditions. The THS3122 is suported by TI's High Speed Amplifiers product group. They have an E2E forum and should be able to assist you with any application questions you may have regarding the op amp.

    Regards, Thomas

    Precision Amplifiers Applications Engineering