# BJT output voltage as function of input voltage

• gfd43tg
In summary, the conversation discusses finding Vout as a function of Vin in a BJT common emitter amplifier circuit. The BJT symbol is replaced with a simplified model and methods are discussed for solving for Vout. The concept of a minus sign in the analysis is brought up but ultimately deemed unnecessary in this particular circuit.
gfd43tg
Gold Member

## Homework Statement

The circuit below is a BJT common emitter amplifier. Find Vout as a function of Vin. This may seem hard to parse at first, but it is actually pretty straightforward. Carefully replace the BJT symbol with the model for a BJT we used in class, making sure the base (B), emitter (E), and collector (C) terminals are connected properly, then use any method you want to solve for Vout!

## The Attempt at a Solution

My solution doesn't use V0 at all. I am wondering if my connection is correct and if I did this correctly.

#### Attachments

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Looks ok for me. And your BJT model is too too simplified to be able to take into account the Vo influence.

Last edited:
Fine, except needs a minus sign.

why the minus sign?

Maylis said:
why the minus sign?

Because if the input goes up, the output goes down.
If your input were a sine wave, the output would be a sine wave of opposite phase.

where does the minus sign come in for the analysis? I don't just want to tack on a negative to the front of my answer based on a concept with no proof

No, you don't need any "minus" sign. In "normal" CE amplifier the output voltage is taken between collector and ground. But in your circuit Vout is the voltage across Rc resistor. So you don't need "minus" sign.

Jony130 said:
No, you don't need any "minus" sign. In "normal" CE amplifier the output voltage is taken between collector and ground. But in your circuit Vout is the voltage across Rc resistor. So you don't need "minus" sign.

That is correct. Why anyone would want to take the output that way I don't understand, but you are correct.

## 1. How does the input voltage affect the output voltage of a BJT?

The output voltage of a BJT (Bipolar Junction Transistor) is directly proportional to the input voltage. This means that as the input voltage increases, the output voltage will also increase.

## 2. What is the relationship between the input and output voltages of a BJT?

The relationship between the input and output voltages of a BJT can be described by the BJT's voltage gain. This is the ratio of the change in output voltage to the change in input voltage, and is typically represented by the Greek letter beta (β).

## 3. Does the output voltage of a BJT change with different input voltages?

Yes, the output voltage of a BJT will change with different input voltages. This is due to the amplifying properties of the BJT, which allows it to increase or decrease the input voltage to produce a higher or lower output voltage.

## 4. How is the BJT output voltage as a function of input voltage graph shaped?

The BJT output voltage as a function of input voltage graph is typically a linear relationship, where the output voltage increases or decreases in a straight line as the input voltage increases. This can be seen in the BJT's voltage gain equation, where the output voltage is directly proportional to the input voltage.

## 5. Are there any factors that can affect the BJT output voltage as a function of input voltage?

Yes, there are several factors that can affect the BJT output voltage as a function of input voltage. These include the BJT's operating conditions, such as temperature and current, as well as any external components or circuitry connected to the BJT. Additionally, the type and characteristics of the BJT itself can also impact the output voltage as a function of input voltage.

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