# Transistor as Current Source. Forward Active?

• fallen186
In summary, the conversation discusses the use of a transistor as a current source and the difficulties in maintaining a constant voltage at the base (V_{B}) to achieve this. It is noted that a transistor with a constant V_{B} (e.g. V_{B}=1V) only needs V_{C} \approx 0.3V to act as a current source, but if V_{C} < V_{B}, the transistor is not in forward active mode and may not function as desired. The use of a resistor to maintain a relatively constant base current is suggested as a solution. Additionally, the conversation mentions the possibility of variations in VBE affecting the performance of the current source.
fallen186

## Homework Statement

Why does a transistor with a constant $V_{B}$ (e.g. $V_{B}=1V$) only need $V_{C} \approx 0.3V$ to act as a current source;

If $V_{C} < V_{B}$ doesn't that mean the transistor is not in forward active mode and thus not a mode we want to work in.

## The Attempt at a Solution

PSpice Code:
Understanding Transistors as Current Sources

.model my_npn NPN(BF=150 NF=1.1 VAF=80V CJC=6P CJE=10P)

*Voltage Sources
VC 1 0 dc 0;
VB 2 0 dc 1;

*Transistor C B E
Q1 1 2 0 my_npn;

.Probe
.OP
.DC VC 0.1 1 0.001;
.end

#### Attachments

• CurrentSourceTransistor.jpg
37.8 KB · Views: 467
So you were allowing base current to reduce (in whatever way it chose to) as Ic increased? You have no idea what the base current is doing here, do you? Try plotting it, too. What you have is an unusual way of making a constant current source (and I'm not sure that we could have foreseen how well it would perform, on paper, anyway). I'd say we don't usually make a controlled current source this way because of the extreme difficulty of holding VBE perfectly constant. Try varying VBE by a tiny amount, and your "constant" current source will probably reveal its extreme vulnerability.

It is easy to hold IB relatively constant by a resistor from a voltage V where V>>VBE.

http://www.du.edu/secs/departments/ece/media/images/TransistorGraph.jpg

Just my thoughts. I could be way off.

Last edited:
I'm hoping to see your plot of base current needed to maintain VBE=1.0 v.

## 1. What is a transistor?

A transistor is a semiconductor device used to amplify or switch electronic signals. It is made up of three layers of doped materials, typically made of silicon, which are sandwiched together to form a P-N junction.

## 2. How does a transistor function as a current source?

A transistor can function as a current source by controlling the flow of electrons or holes through the semiconductor material. In the forward active region, a small base current is used to control a larger collector current, creating a constant output current source.

## 3. What is the forward active region of a transistor?

The forward active region is the operating state of a transistor where it is used as an amplifier. In this region, the transistor is biased with a positive voltage at the base and a larger output current is allowed to flow from the collector to the emitter.

## 4. How does a transistor maintain a constant output current in the forward active region?

In the forward active region, a transistor maintains a constant output current by using a small base current to control the larger collector current. This is achieved through the current amplification properties of the transistor, where a small change in base current results in a larger change in collector current.

## 5. What are some applications of a transistor as a current source in the forward active region?

Transistors in the forward active region are commonly used in audio amplifiers, voltage regulators, and other electronic circuits that require a constant and stable current source. They are also used in digital logic circuits, where they act as switches to control the flow of current.

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