# Arrow-notated resistances in transistor circuits

Discussion in 'Homework Help' started by ElectricMagician, May 18, 2015.

1. ### ElectricMagician Thread Starter Member

Jul 26, 2012
57
0
Hello,

Can any one give me a clear, generic way to understand what resistance these arrow-notated resistors found in amplifier circuits are supposed to express?

Also, in the example below, is Rin
1) (Vi - VRB1)/(Current through RB1)
2) The above paralleled with re+Re
3) Something else

As to Rout, is it
1) RE
2) RE paralleled with (re + part 1 above)
3) Something else

(this is not the actual problem requirement, but I find this notation highly counter intuitive so I wanted to make sure I correctly understood what is required before attempting to solve it)

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2. ### WBahn Moderator

Mar 31, 2012
20,245
5,758
It means the resistance seen looking into that port.

If you increase Vi by some amount, ΔV, you will increase the current flowing out of Vi by some amount, ΔI. If you take the ratio of these, you will get a resistance. That resistance is the series sum of Rb1 and Rin.

So imagine removing everything beyond the tip of the arrow and replacing it with a resistor (and possible a source to establish the DC operating point) and asking what value of resistance will match the slope of V/I characteristic at that point.

3. ### ElectricMagician Thread Starter Member

Jul 26, 2012
57
0
So the expression for Rin in this case will be the internal resistance in the transistor + Re, whereas the expression for Rout will involve VCC, Vi and the resistances in between. Is this correct?

4. ### WBahn Moderator

Mar 31, 2012
20,245
5,758
It would be best if you showed your work and then we can look it over and provide feedback.

5. ### crutschow Expert

Mar 14, 2008
16,551
4,464
The value of Rin also involves the current gain of the transistor.

6. ### ElectricMagician Thread Starter Member

Jul 26, 2012
57
0
The solutions to the simple questions I asked would have significantly helped me understand and saved me a lot of time.

Thank you

7. ### crutschow Expert

Mar 14, 2008
16,551
4,464
Since this is the Homework Help forum not the Do Homework forum that is what we normally do.
Sorry if that's too time consuming for you.
Doing a search for "common emitter amplifier input impedance" I found this tutorial which may help.

8. ### panic mode Senior Member

Oct 10, 2011
1,423
348
arrow simply points towards part of a circuit... if you cut out part of circuit where arrow points and replace it with some equivalent resistance, what would this equivalent resistance be? It is Rin or Rout of course... (in and out is here merely description).

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