# Largest resistor value such that BJT remains in active mode

• theBEAST
In summary, the conversation is discussing a transistor with a given beta value and a figure showing a solution. The question is raised about the inequality Vc - Vb > 0.4V and whether it should be Ve - Vc > 0.2V based on the concept of saturation. The conversation then shifts to determining the voltage between C and B at saturation for a silicon transistor.
theBEAST

## Homework Statement

We are given that the transistor has β = 30. The BJT is shown below (along with the solution). Every value in the figure is given with the exception of the equation I circled in red:

## The Attempt at a Solution

What I don't get is how they got the inequality Vc - Vb > 0.4V? Where does this come from? I thought saturation occurs when Vce reaches 0.2V, so shouldn't our inequality be Ve - Vc > 0.2V?

Assume that the magnitude of the base-emitter voltage for the BJT is largely constant. What value might you choose?

theBEAST said:
I thought saturation occurs when Vce reaches 0.2V?
You are probably right. So at saturation what would be the voltage between C and B? Work it out for a silicon transistor.

## 1. What is the definition of "active mode" for a BJT?

The active mode, also known as the linear mode, is a state of operation for a bipolar junction transistor (BJT) in which the transistor amplifies a signal without significant distortion. In this mode, both the base-emitter junction and the base-collector junction are forward biased.

## 2. How is the largest resistor value for active mode determined for a BJT?

The largest resistor value for active mode is determined by the maximum collector current (IC) and the minimum base current (IB) required to keep the BJT in active mode. This is known as the DC current gain (hFE) of the BJT and is typically specified in the transistor's datasheet.

## 3. What happens if the largest resistor value is exceeded for a BJT in active mode?

If the largest resistor value is exceeded, the base current (IB) may become too small to maintain the BJT in active mode. This can result in the BJT entering either the saturation region, where the collector current becomes independent of the base current, or the cutoff region, where the collector current becomes zero.

## 4. How does temperature affect the largest resistor value for active mode in a BJT?

The largest resistor value for active mode is affected by the temperature due to the temperature dependence of the BJT's DC current gain (hFE). As the temperature increases, the hFE decreases, which results in a lower maximum resistor value for active mode. Therefore, it is important to consider the operating temperature when determining the largest resistor value for a BJT in active mode.

## 5. Can the largest resistor value for active mode be increased by using a Darlington pair configuration?

Yes, the largest resistor value for active mode can be increased by using a Darlington pair configuration, which consists of two BJTs connected in a common-emitter configuration. This configuration has a higher DC current gain (hFE) compared to a single BJT, allowing for a larger maximum resistor value for active mode. However, this also introduces a higher input capacitance and a longer turn-off time for the BJT pair.

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