How to measure the Slew Rate

[flamingo]

Given an amplifier, how to simulate the SR in Spice. for example, amplifier's freq about 40MHz, what about the input pulse signal, such as peroid or frequency ?

 

[batdin]

I would drive this amplifier with a square-wave signal with a freq. for example 100kHz.Then I would measure its slew rate by displaying the first derivative of Vout.
The peak values should be what you need.

 

[flamingo]

I would drive this amplifier with a square-wave signal with a freq. for example 100kHz.Then I would measure its slew rate by displaying the first derivative of Vout.
The peak values should be what you need.

why 100KHz, how to decide the range of input frequency? I had experienced bad output when using the square-wave input as T=50ns, that is freq=20MHz (the amplifier worked in 40MHz input sinewave), if I increase the peroide to about 50us (freq=20KHz), the output is normal, i,e, the output is the amplified square-wave with small distortion.
So i dn't know how to choice the exact input frequency, and what the influce of different input square-wave?

 

[jordan76]

Hi,

Slew Rate is defined as the maximum output voltage rate(either positive or negative). For typical two stage amplifier, SR=Imax/C,where Imax is the tail current and C is the compensation capacitor.

If you choose sine wave input signal, you need to make ω larger than SR/A (A is the sine wave amplitude). Similarly,if you choose square wave input signal, you need to make the rise/fall slope larger than SR.

hope it helps.

regards,
jordan76

 

[batdin]

Hi again,
100 kHz is more or less a random choice.
It could as well be 30.2435 kHz
But if it's too high,the amplifier will behave strangely.
If it's too low,the spikes you need to measure will be very few.
40MHz is the small-signal unity-gain bandwidth of the amplifier.
It won't work correctly at that frequency.
The amplitude of the input square-wave signal should be enough to provide the necessary output level (let's say Vcc-3V) but not to saturate the amp.