Yes, an RC high pass filter on the input
will increase the noise. Although the capacitor doesn't itself add
any noise, it causes the effective input noise voltage to
increase, because:
Vn(tot) = ¡Ì(En^2 + (In/2/pi/Cser)^2)
..where: En = I/P noise voltage density, and In = I/P noise
current density (neglecting the source resistance noise, which
often you can't do)
Remember also: "No attenuation before gain".
I should also mention (without seeing your schematic), that many
of the devices in the LTspice standard libraries don't have
realistic noise parameters, especially when it comes to 1/f noise.
--
Regards,
Tony
On 20/02/2025 20:17, manauo via
groups.io wrote:
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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?
Tony Casey