# BJT amplifier: small-signal analysis using H-parameters

I'm preparing for an exam and I have this problem where I need to find the input resistance as it is seen by the voltage generator ug.

After using H-parameters to find an equivalent circuit I came up with this equivalent schematic:

Can anyone explain what it means to find the input resistance (denoted by Rul) in my schematic as it is seen by voltage generator ug? What is the difference between "normal input resistance" and this input resistance?

• There are two definitions of gain. The first is versus the generator itself. The other includes the internal generator impedance with Rg versus Vb. The impedance are then Rul =(Ug/Iul) or (Rul-Rg)= Vb/Iul. Commented Jan 21, 2023 at 17:49

I had initially said that the resistor $$\R_E\$$ needed to be included. However, the capacitor in parallel most likely shorts it for signal frequencies of interest.

If you use an common-emitter amplifier without feedback, then its DC input resistance is $$\R_b + R_\pi\$$ (I'm thinking of the BJT small-signal pi-model). However, if you add some sort of feedback, like in your example (it's series-series feedback), then this will change the nominal input resistance of your BJT.

So, you could say there is an "open-loop" type of input impedance, which I already mentioned for the BJT. However, when you use any kind of feedback, this changes the input impedance. If you use series feedback at the input, that tends to increase the input impedance, while shunt feedback tends to lower it (e.g. a transimpedance amplifier uses shunt-feedback at the input as well as the output).

• I didn't include the Re resistor because of short-circuiting Cp capacitor. I think I should short-circuit that capacitor because its impedance is 0. Commented Jan 21, 2023 at 17:09
• You're right, but the 2nd part of my answer still holds. Commented Jan 21, 2023 at 17:10

For reference ...

Here is an example of "calculating" input impedance (with Rg or not).
The impedances are Rul =(Ug/Iul) or (Rul-Rg)= Vb/Iul.

The question seems to be about what the question means. If it is not clear what the question means that is a fault of the questioner.

If asked "resistance seen by the generator" one would expect to disconnect the generator and then find the resistance at that terminal pair. The resistor RG maybe is part of the generator? It position and the choice of symbol at least implies this.