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Consulting some questions about amplifier

user4768469
Intellectual 290 points
Other Parts Discussed in Thread: OPA1692, OPA172, TLV9062

1.For bipolar amplifier, we consider the output impendance is resistive, it is not variable with frequency, is it true? But for CMOS or JFET amplifier,the output impendance is changed with frequency,yes or not?

2.Are all the amplifiers a single pole system?

3.we know that slew rate limits bandwidth, Fmax = (Slew Rate)/2πVpk，we get the formula from sinewave input signal, but for other input signals,such as square,triangle and arbitrary signal and so on, what's the relationship between slew rate and the full power bandwidth,Fmax?

Thanks!

  • 0
    Guru 140200 points

    Hi,

    you might want to read this:

    slyt677.pdf

    Kai

  • 0
    TI__Expert 6625 points

    Hi User,

    Please find my comments below. 

    1. For bipolar amplifier, we consider the output impedance is resistive, it is not variable with frequency, is it true? But for CMOS or JFET amplifier,the output impedance is changed with frequency,yes or not?

    No, the output impedance being resistive or reactive is independent of the transistor technology. The output impedance is dependent on the overall design architecture. 

    For example, OPA1692 is a bipolar amplifier but as you can see from the datasheet the output impedance is reactive. 

    OPA172 is a CMOS amplifier but the output impedance is reactive.

    TLV9062 is a CMOS amplifier but the output impedance is resistive.

    2 .Are all the amplifiers a single pole system?

    No, they've multiple pole-zero pairs. The frequency where this happens is dependent on the application that the amplifier is designed for. 

    3. We know that slew rate limits bandwidth, Fmax = (Slew Rate)/2πVpk，we get the formula from sine wave input signal, but for other input signals,such as square,triangle and arbitrary signal and so on, what's the relationship between slew rate and the full power bandwidth,Fmax?

    To explain this I will talk about 3 common input signals:

    1. Square Wave - I don't think there is a formula that relates slew rate to Fmax for a square wave. But, please note that the slope of a square wave is close to infinity but the slew rate of any amplifier is limited. You need to make sure that the half time period (1/2f) is always greater than the slew time (time taken to ramp up to Vpeak). If this requirement is not met then you will start to notice slew induced distortion. 

    2. Sine Wave - You can use the formula stated in your question for this case. 

    3. Triangle Wave - This input signal is the least affected due to slew rate. The output will be fine as long as the slope of the triangle wave is lesser than the slew rate of the device. 

    To summarize square waves will be the most affected by slew rate bandwidth limitation and the triangle wave is the least affected. 

    Best Regards,

    Bala Ravi