Hello,
?
I don't have much experience using LTSPICE for noise simulations, and I'm having trouble understanding some of the results I'm getting. I'm investigating an amplifier circuit (file "Amplifier Noise" uploaded to files/Temp).
?
The amplifier is a cascode differential pair, signal frequency is 1MHz. Currently the circuit has no filtering besides the inherent roll-off at high frequency. I'm investigating whether adding AC coupling capacitors to the input to filter low-frequency noise can improve the performance. The inductance/resistance in series with each input represents the output impedance of the previous stage.
?
When I simulate the original circuit with no coupling capacitors, the result seems reasonable - relatively flat, input noise is a fraction of the output noise. However when I simulate the proposed circuit with coupling capacitors I don't understand the result.
?
The input noise balloons at low frequencies - the frequency range that the HPF of the capacitor is supposed to reduce, is now much greater. The output noise is lower in terms of total integrated noise, but still has an increased magnitude at low frequency compared to the original.
?
The decrease in total output noise is coming from a decrease in the high frequency noise, which makes sense because the inductance of the source impedance and the base pull-down resistor form an RL LPF. But the capacitor does not seem to be reducing LF noise, even though an AC sweep shows the expected HPF behavior.
?
Am I misinterpreting the results somehow? How is adding a high-pass filter at the input increasing noise at the low frequencies that it is supposed to be attenuating? What am I missing about noise simulations?
