# Solving simple transistor circuit

• Engineering
• davidbenari
In summary, the equations for calculating the current and voltage at the base-emitter junction are contradictory.
davidbenari

## Homework Statement

Determine IEQ ICQ IBQ

My problem is that this circuit is overdetermined and cannot be solved. (See below)

##\beta = 75##

##I_E=76 I_B ##

## The Attempt at a Solution

The current IB is calculated as ##I=\frac{1}{500}=.002mA##

The current ##I_E## is ##I_E = 76*.002 = .152 mA##

Similarly ##I_C= 75 * .002 = 0.15 mA##

The voltage ##V_{CE}## is calculated by knowing the current in the emitter and starting upwards from the -3V node, in other words

##V_{CE}=3-(-3+4.8*.152)=5.27V##Knowing this we see that there is a contradiction. The voltage drop across the BE is not .7 .

Is this a sound way of proving that the BE voltage doesn't HAVE to be 0.7?

Typically you don't know the beta of the transistor very accurately. A better assumption is probably that Vbe = 0.7 V, then calculate what the voltage is across the 4.8 kOhm resistor, then calculate IEQ, then beta. Beta appears to be significantly higher than 75.

davidbenari
Oh okay. Its just I was given the value of beta.

What I've noticed is that BE voltage is always taken as 0.7V (or similar), however in saturation mode when both BC and BE are forward biased, BE is 0.7 while BC isn't necessarily 0.7 (I've seen 0.5 in problems). This seems weird to me since forward biased is considered as 0.7 or above. Is this normal?

davidbenari said:
Oh okay. Its just I was given the value of beta.

What I've noticed is that BE voltage is always taken as 0.7V (or similar), however in saturation mode when both BC and BE are forward biased, BE is 0.7 while BC isn't necessarily 0.7 (I've seen 0.5 in problems). This seems weird to me since forward biased is considered as 0.7 or above. Is this normal?
A base-emitter voltage of 0.7 V is a handy approximation suitable for typical design and analysis work. But it is just a "rule of thumb" sort of approximation, pretty good for silicon transistors under typical operating conditions. Not so good at all for germanium or other types of transistors.

The base-emitter junction is essentially a diode, and as such its forward voltage will vary like a diode; the IV curve is not a perfect vertical line at 0.7V. You may see forward voltages between 0.6 and perhaps 1.3 or so volts under different circumstances. Look up the "diode equation" .

## 1. What is a transistor?

A transistor is a semiconductor device that is used to amplify or switch electronic signals. It is composed of three layers of material, typically silicon, with two layers of either n-type or p-type semiconductor sandwiching a layer of the opposite type.

## 2. How does a transistor work?

A transistor works by controlling the flow of electrons between the two outer layers, called the collector and emitter, with the middle layer, called the base. By applying a small voltage to the base, the transistor can either amplify or switch the current flowing through the device.

## 3. What is a simple transistor circuit?

A simple transistor circuit is a circuit that uses one or more transistors to control the flow of current, typically to switch on or off a larger current flow. It can be composed of a few basic components, such as resistors, capacitors, and a power source, in addition to the transistor(s).

## 4. How do I solve a simple transistor circuit?

To solve a simple transistor circuit, you will need to understand the basic principles of how a transistor works and how it is connected within the circuit. You will then need to use circuit analysis techniques, such as Kirchhoff's laws and Ohm's law, to determine the voltage and current at different points in the circuit. Finally, you can use these values to calculate the overall behavior of the circuit.

## 5. What are some common applications of simple transistor circuits?

Simple transistor circuits have a wide range of applications, including amplification of audio signals in speakers, switching on and off electronic devices, and controlling the flow of electricity in various electronic devices. They are also commonly used in digital logic circuits, such as in computers and smartphones.

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