Stability of a single stage amplifier (source follower or common emitter)

[Jim cage]

Hi,

Is there anyway to simulate the PM of a single stage amplifier using iprobe?
Does anybody knows a good paper on a stability of a single stage transistor amplifier (and not in terms of S-parameters).

BR

 

[wanghs1986]

close loop, add iprobe (or a plane voltage source), run stb

 

[leo_o2]

Yes, please use STB analysis in cadence ADE. For hspice, you have to do AC analysis.

 

[WimRFP]

Hello,

You can assess stability based on the loop gain and via negative resistance, if you don’t want to use the S-parameter approach.

For active components the loop gain method is difficult to use in practice as the feedback loop is via parasitic components inside the active component. In simulation it is possible, but you have to "open" the active device.

Negative resistance method
When you look into the input of an amplifier (gate or base), input impedance (Zi) varies with frequency. When the input impedance becomes negative in certain frequency regions, there is a risk on instability.

If, in the frequency region where Re(Zi)<0, Im(Zi) doesn't change sign, the amplifier will be stable. When it changes sign 1, 3, 5, etc times, it will be unstable under certain input conditions.

In simulation, a current source is easy to apply.
You need to put your AC current source as close as possible to the input of the active component. The other terminal goes to ground. Measuring the voltage across the 1A current source gives you the input impedance directly (AC small signal simulation). When there is attenuation between your "measuring point" and actual input of active device, you will not see the actual negative resistance.

It is nice to simulate the common drain or common collector topology. When the load becomes capacitive, you will find regions where Re(Zi)<0. As long as Im(Zi) doesn’t change sign, it will be stable. Adding an inductor between base and ground may result in change of sign of Im(Zi) in the region where Re(Zi)<0. In that case you made an oscillator.

Common emitter or common source will show negative input impedance when the output (collector, drain) becomes inductive. When the base sees inductive source impedance, the circuit may become unstable. When you put your current source between base and ground, you will notice instability based on Zi versus frequency.