# Am I confusing input & output impedance?

• iScience
In summary: If the transistor is saturated, then the collector current would be infinite, and the transistor would be destroyed. This would not be a problem in a switch, because the transistor would be destroyed before it reached the switch.
iScience

## Homework Statement

design a common-emitter amplifier with an output impedance of 4.7kOhms and again of-10 using a transistor with a beta=200 powered by a 12V DC supply.

v=ir

## The Attempt at a Solution

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rest of the problem i get; just this one thing bugging me.
the solution starts out with solving for the collector current. I understand why the output is taken at the collector, but i don't get why they set R_c equal to the given output impedance. i thought the output impedance was the equivalent resistance as seen from where ever the "output" was being taken (in this case the collector).

Perhaps think of a amplifier as a voltage source that has some internal resistance (capacitance or inductance). The output impedance is a measure of how near the output is to an ideal voltage source. If the output impedance is very low then the load (the input impedance of the next stage) will have little effect.

The output impedance of a common emitter amp is roughly equal to the collector resistor. The required output resistance was specified as 4.7K so they set the collector resistor at that value.

CWatters said:
The output impedance of a common emitter amp is roughly equal to the collector resistor. ...

...because the exact output resistance is ro||Rc. And the internal BJT output resistance ro is very large (20...50 kohms).

but if we were in saturation mode would we take into account r0?

Why would the transistor be saturated? It should be in a linear mode.

high power switch?

Well yes in a switch the transistor might be saturated but normally you would hope it's not saturated in an amplifier. What happens to the output if it does?

iScience said:
but if we were in saturation mode would we take into account r0?

## What is input impedance?

Input impedance refers to the electrical resistance of a circuit or device that is connected to the input of a system. It is typically measured in ohms and is used to determine the amount of voltage that will be required to produce a certain amount of current in the input circuit.

## What is output impedance?

Output impedance refers to the electrical resistance of a circuit or device that is connected to the output of a system. It is also measured in ohms and is used to determine the amount of current that will be produced when a certain voltage is applied to the output circuit.

## What is the difference between input and output impedance?

The main difference between input and output impedance is their location in a circuit. Input impedance is measured at the input of a system, while output impedance is measured at the output. Additionally, input impedance is used to determine voltage requirements, while output impedance is used to determine current production.

## Why is it important to understand the difference between input and output impedance?

Understanding the difference between input and output impedance is important because it allows you to properly design and troubleshoot circuits and devices. It helps ensure that the correct voltage and current levels are being used, which can affect the performance and efficiency of a system.

## How can I determine if I am confusing input and output impedance?

If you are unsure whether you are confusing input and output impedance, you can double check by looking at the location of the impedance measurement and the type of information it is providing. If it is measured at the input and is used to determine voltage, it is input impedance. If it is measured at the output and is used to determine current, it is output impedance.

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