# Finding Rout Rin in schmitt trigger

#### alibedelci

Joined Apr 15, 2013
10
Hi, I have a lab assignment which I'm struggling with for about 6 days now, and it's almost complete, except:

What are the theoretical input and output impedances of the circuit in terms of the resistances R1,
R2, R3, R4 and RE ? The voltage specifications of your Schmitt trigger (the values Vturn-on, Vturn-off , Von
and Voff you report) actually depend on the input and output impedances of your trigger, the input
impedance of the load, and output impedance of the driving circuit. If you would like your circuit to
adhere to these specifications to within 1%, estimate (a) the upper limit on output impedances of
circuits used to generate input signals for your Schmitt Trigger, and (b) the lower limit on input
impedances of loads driven by your Schmitt trigger. Do the function generator and oscilloscope you
used to evaluate the circuit fall within these limits?

#### WBahn

Joined Mar 31, 2012
30,248
Well, if you've spent six days struggling with it you should have a fair amount of work to post for us to use as a starting point. Please post your best effort to date.

#### alibedelci

Joined Apr 15, 2013
10
@WBahn

Hi, I have spent six days overall for all lab questions. I just do not know how to calculate Rin/Rout etc. I can post my lab report if you like, but I think that's besides the point and has no relevance to the issue at hand right now.

Thanks,

#### WBahn

Joined Mar 31, 2012
30,248
You need to give us something relevant to start with. Start with a description of what you think Rin and Rout are and how you might go about calculating them.

#### alibedelci

Joined Apr 15, 2013
10
I think Rin is looking from base into emitter of the first Q1, but I think because there are two transistors cascaded, I should include (or should I?) Rin of second (again from looking base to emitter of Q2 this time) to the equation. I definitely think that the Rin of second transistor includes Rout of the first one.

Rin can be calculated by Vin/ib but this is all theoretical talk from an inept student, unfortunately.

#### #12

Joined Nov 30, 2010
18,224
Re-draw the circuit as a differential pair and you will see how Q2 affects the input impedance.

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