Understanding why R_E is in parallel with g_m in a BJT

In summary, the conversation discusses the analysis of a circuit and the role of certain parameters such as R_E, 1/g_m1, V_A, and r23. The speaker also mentions using the small signal model and the assumption of β → ∞ in their analysis. There is also some confusion about the relevance of gm3 in the circuit.
  • #1
kl055
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Homework Statement


Why is R_E in parallel with 1/g_m1?
I drew the small signal model but it does not make sense to me


Homework Equations


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The Attempt at a Solution

 
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  • #2
I get that V_A is given by a voltage divider, but why is R_E in parallel with 1/g_m1?
 
  • #3
Q3 is just a diode so gm3 can't play a role.

What is r23? What is VA? You need to define some of these parameters. I assume gm = β but am not sure ...

I analyzed the circuit assuming β → ∞. Why? Because any circuit that is β-dependent is not a good circuit.

So I got what looks like your expression if gm → ∞. I know that's not what you are trying to find out - sorry.
 

1. What is RE and gm in a BJT?

RE is the emitter resistance and gm is the transconductance of a BJT (bipolar junction transistor). RE is the resistance between the emitter terminal and ground, while gm is a measure of the BJT's ability to amplify current.

2. Why are RE and gm in parallel in a BJT?

In a BJT, the emitter current is controlled by both RE and gm. RE provides a fixed path for the emitter current, while gm varies the current based on the input signal. Therefore, RE and gm are in parallel because they both contribute to controlling the emitter current.

3. How does RE affect the performance of a BJT?

RE affects the DC biasing of a BJT, as it sets the operating point for the transistor. A higher value of RE will result in a lower emitter current, while a lower value will result in a higher emitter current. This can affect the gain and linearity of the BJT, as well as its stability.

4. What is the significance of gm in a BJT?

gm is a key parameter in determining the small-signal gain of a BJT. It also affects the input and output impedance of the transistor. Additionally, gm plays a role in the frequency response of the BJT, as it can limit the high-frequency performance.

5. How can the understanding of RE and gm in a BJT be applied in practical circuits?

Knowledge of RE and gm can be applied in the design and analysis of BJT amplifiers. By manipulating the values of these parameters, the performance of a BJT circuit can be optimized for specific applications. Additionally, understanding the relationship between RE and gm can aid in troubleshooting and improving the stability of BJT circuits.

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