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THS4509: THS 4509 for gain and impedance matching

vinay bhatt
Prodigy 220 points
Part Number: THS4509
Other Parts Discussed in Thread: THS4505, THS4504

The circuit is as follows:

codec giving differential output of 800 mVrms, then THS4509 for amplification and impedance matching. Resistance connected between THS4509 and codec is 100 ohm. The required output after THS4509 is 8.5 Vrms, 600 ohms. The gain required from THS4509 is 20 log (8.5/800 mV) = 20.52 dB

How do I select resistance values?

  • 0
    TI__Expert 6050 points

    Hi Vinay,

    Unfortunately, the THS4509 will not be able to be used for this application because it has a maximum supply voltage of 5.25V with an output voltage below that. Therefore, the desired output of 8.5Vrms is not achievable with this part. I can help you pick an amplifier that will suit your application, though. If you could provide me the information below, I can provide the best recommendations:

    • What is the frequency of the signal being input into the amplifier
    • Is the amplifier output differential or single ended
    • What are your power supply capabilities (in order to achieve an output of 8.5Vrms, you will need a power supply around 15V)
    • Are you running the amplifier in a single or dual power supply configuration
    • What is the voltage range of the amplifier input voltage

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 220 points

    Thanks Nick

    1.  frequency will be in voice range - 300 to 3300 Hz

    2. amplifier is differential

    3. Max power supply is 3.3 V , but if required can be changed

    4. Amplifier is being run at dual power supply configuration

    5. amplifier input voltage range - by this you mean signal to be amplified? then 0 to 800 mVrms.

  • 0 in reply to vinay bhatt
    Prodigy 220 points

    point 4 - single power supply configuration

  • 0 in reply to vinay bhatt
    TI__Expert 6050 points

    Hi Vinay,

    Thanks for providing this information! Based on this information, I recommend using the THS4504 or THS4505 (THS4504 has power down capability). I have included a circuit simulation so you can see the behavior and make any adjustments to fit your exact circuit needs.

    To answer your original question:

    The feedback resistors are typically values that can be found on the datasheet based on the gain you are trying to achieve. Using the values defined on the datasheet will help with stability. Then the gain resistor can be found by dividing the feedback resistor value by the gain.

    THS4504_E2E.TSC

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 220 points

    Thanks Nick

    600 ohm is load resistance, there will be characteristic impedance of trace between load and op amp Ro. How to match the impedance of load with R0?

  • 0 in reply to vinay bhatt
    TI__Expert 6050 points

    Hi Vinay,

    This article (link) explains how to match the output impedance (Ro) of the opamp to the load resistance when there is an impedance due to a transmission line. Please let me know if you have any other questions.

    Thanks,

    Nick