AC and transient simulation

[wafi_zuhdi]

Could someone shed a light on me for this. i'm running an AC analysis on feedback loop, and the bandwidth of the system is 200Mhz.

When i run transient simulation, supposedly i put in a sine wave, i wont be expecting it to be attenuated if the sine wave freq is less than 200Mhz . is that right? but my output signal starts to attenuate if the sine wave signal > 100Mhz. and also it varies if i change the magnitude of the sine wave, the bigger the sinewave (peak-to-peak), the faster it gets attenuated. what have i done wrong here? or did i get the concept wrong?

 

[LvW]

Your problem is not clear to me.
For example: i'm running an AC analysis on feedback loop, and the bandwidth of the system is 200Mhz.


What are you simulating? A closed-loop system? The loop gain ? What means "bandwidth": from 0 Hz to 200 MHz? Or 200 MHz centered at Fo?

The best method to ask a question is to show us a circuit diagram of the simulation arrangement.

 

[wafi_zuhdi]

Hi LvW, thanks for the reply. I'll try to be more clear. hope this helps

i'm doing a closed loop system. so the 3dB-bandwidth of the closed-loop system is around 400Mhz. so the question is when i put a sine wave in with amplitude 100mV p-p, it works fine until its freq around 400Mhz. But if i increased it, say to 700mV, it starts to attenuat around 100Mhz. i attached part of the schematic. hope you could understand.

 

[LvW]

At first, I cannot detect the type of FET and the phase characteristic of the transistor circuitry (inverting/noninverting?).
At second, what is the gain and the supply voltage(s) of the complete circuit? Perhaps there is an overdrive effect at 700mV ?

 

[wafi_zuhdi]

it's an OP-amp driving a PMOS. By right the feedback should a unity gain, as voltage drop across the resistor should be able to track changes in Vin (negative input). it's uses a 3.3 V power supply. Am i right to think that AC simulation only gives an indication of what the small signal analysis would be like. Because at small amplitude input, the transient and AC gives about the same result. Therefore i think my problem is that the op-amp may not be strong enought to drive a high amplitude input?

 

[supreet_95]

Hi,
The loop has a negative feedback which ensures that output node (drain of pmos) follows the input value. However this only the case if it is "biased" properly. When we apply small-signal or sine wave of small amplitude we do not disturb the quiescent point of the system, where the gain was measured to be high. However when the input sine wave has higher amplitude, we start to move the quiescent point itself, like decreasing the Vds of pmos to bring it into "linear" region, which decreases the gain and attenuates the signal. The application (input/output common mode range) should make sure not to move the quiescent point of system, which could alter the characteristics and add non-linearity into system like variable gain. Hope this helped.

 

[sutapanaki]

Most probably you are experiencing slew rate limitation in the amplifier. This would explain why you see reduction in the output amplitudes when you increase the input. Do you see distortion in the output sine wave? Check the current in the input differential transistors and see if at the peaks of the input signal one of the differential transistors gets all of the tail current. Another possibility could be decreasing too much the Vds voltage across the tail current transistor at the voltage extremes or decreasing the Vds across the output PMOS transistor.