I am going through an older textbook and am looking at some examples regarding stability. I can successfully model the circuit in the book and get the same results as the author. In this case the author provided SPICE parameters, which made this example attractive to try out. The schematic from the text is shown below


simulate this circuit – Schematic created using CircuitLab

As I said I successfully modeled the stability of the amplifier by opening the loop and loading down the input and output of the amplifier based on the feedback impedance. The simulation matched the book results, and is shown below in LTSpice.

enter image description here

Getting this far I thought it would be interesting to compare this technique in determining loop gain versus inserting an AC signal in the feedback path. However, when I try this the gain inevitably begins to increase beyond 1MHz (and the phase with it). The best I can do is to insert the voltage source after the emitter of the second transistor to match the result from the text (and my own simulation).

enter image description here

For reference, I looked at a previous post but am uncertain if it is applicable. Can anybody tell me whether the placement of the source is correct (or if there is a better location), or why the response rises in gain above 1MHz?

  • \$\begingroup\$ Is anybody able to comment on this? \$\endgroup\$ – DWT Mar 22 at 23:35

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