BJT: Common Emitter Amplifier Gain

• tomizzo
In summary, there is confusion about how Vbb can be removed when analyzing the gain of a common emitter amplifier. The attached diagram states that lower case variables are changing values, indicating that this is an AC analysis. Bypassing some or all of the external Remitter with a capacitor can increase the gain, and understanding this concept is key to mastering the common emitter amplifier. It is recommended to tinker with the equations and intuitively understand the circuit instead of just memorizing formulas.
tomizzo
So I have a question regarding the gain of common emitter amplifier. Referring to the attached diagram, the voltage gain of the circuit is -RC/RE. However, I don't understand how Vbb can be removed when completing the analysis. I've done the KCL analysis by hand and keep getting a gain function that is dependent on that value of Vbb. But the analysis completed below simply removes that Vbb.

Is the analysis completed in the diagram AC analysis? Therefore the Vbb is removed since it is a constant DC value? Can anyone help clear this up?

Thanks!

Edit: Well it appears that I didn't read the diagram close enough. It states that lower case variables are those of changing values, which I assume means this is AC analysis. Confirmation of this would still be appreciated though.

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Yep that's it. You generally operate over a small part of the curve. But it's not uncommon to include the transistor's internal remitter.

That's why bypassing some or all the external Remitter for AC with a capacitor gives you so much more gain. Bypassing just some of it gives you more predictable gain - when you can explain that to somebody you'll have mastered the common emitter amp.

So tinker with those equations and work the circuit in your head until your intuition comes to agree with them. That sure beats just memorizing them.

1. What is a BJT common emitter amplifier?

A BJT common emitter amplifier is a type of electronic circuit that uses a bipolar junction transistor (BJT) to amplify a small input signal. The BJT is configured in a common emitter configuration, where the input signal is applied to the base terminal and the amplified output signal is taken from the collector terminal. This configuration offers high voltage gain and high input impedance.

2. How is the gain of a BJT common emitter amplifier calculated?

The gain of a BJT common emitter amplifier is calculated using the formula Av = -gm * RC, where Av is the voltage gain, gm is the transconductance of the BJT, and RC is the collector resistor. The transconductance is dependent on the DC biasing current and the characteristics of the BJT. The collector resistor is chosen to provide the desired gain and stability for the amplifier.

3. What are the factors that affect the gain of a BJT common emitter amplifier?

The gain of a BJT common emitter amplifier can be affected by several factors, including the DC biasing current, the characteristics of the BJT, the choice of collector resistor, and the frequency of the input signal. In order to achieve a high gain, it is important to choose appropriate values for these factors and to ensure that the amplifier is operating within its linear region.

4. What is the relationship between the gain and the frequency response of a BJT common emitter amplifier?

The gain of a BJT common emitter amplifier is dependent on the frequency of the input signal. As the frequency increases, the gain of the amplifier decreases due to the internal capacitances and inductances of the BJT. This effect is known as the frequency response of the amplifier. In order to maintain a constant gain over a wide range of frequencies, additional circuit elements such as capacitors and inductors can be added to the amplifier.

5. How can the gain of a BJT common emitter amplifier be increased?

The gain of a BJT common emitter amplifier can be increased by increasing the DC biasing current, choosing a BJT with higher transconductance, and using a smaller collector resistor. Additionally, using a multi-stage amplifier configuration can also increase the overall gain. However, it is important to note that increasing the gain of an amplifier may also introduce other issues such as decreased stability and increased distortion.

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