A phenomena when measuring impedance with feedback technique

[lean]

feedback techniques

Hello everybody. I would like ask you for a curious phenomemnon by
measuring
the output impedance in a circuit. Take a look to this
(www.eurobotics.org/vco/circuito.doc) simple circuit. When I meassure
the
output impedance using the convencional way, that is, using a voltage
source
at the output while shortcutting the dependences ones I get 6 MOhmns. If
instead of this I use feedback techniques to calculate the output
impedance I
get 30 GOhmns... What is going on?.

Thank you all.

 

[flatulent]

Re: feedback techniques

Your manual calculation is not considering the impedance between the collector and base of the BJT. Since the op amp output impedance is almost zero, this transistor impedance (R sub mu) is effectively between the circuit output terminal and ground.

To test this idea, put a second BJT in parallel with the one you have and see if the simulated impedance drops by half.

 

[lean]

Re: feedback techniques

Hello everybody.
If transistor is a Mosfet.
When I meassure the output impedance using the convencional way, that is, using a voltage source
at the output while shortcutting the dependences ones I get 60 GOhmns. If
I use feedback techniques to calculate the output
impedance I
get 60 GOhmns.
when I work with BJT the values are different.
What happen with rpi and ru?

Thank you all.

 

[devrimaksin]

feedback techniques

I dont think the problem is related to base-collector
resistance. 6MOhm Rmu is toooo low for a real device. It might be related to DC setup of your simulation. When you say shorting the input,
you have to keep the DC signal level but remove
the AC component. For proper operation in AC,
you have to put 100mV DC voltage at the input of the amplifier, this will force the positive terminal of the resisitor to be equal to 100mV, hence you will bias your BJT with 100m/4K, 25uA collector current.
Your collector voltage should be of course couple of hundered milivolt above base voltage to assure forward active region operation, so you should choose you collector voltage arround 0.8-1V. Then,
the output resistance is defined as small signal
(that means, if you choose collector bias voltage 1V, your output reistance is not 1V/25uA (bias current))
To measure small signal resistance you have to apply a small signal AC signal (choose a magnitude that guarantee VCB>0) and you have to measure the current flowing through.

 

[lean]

Re: feedback techniques

In Mosfet:

When I meassure the output impedance using the convencional way,

Vx/ix= RE +ro(1 + gmRE(1+Ad))= 60Gohmios

If I use feedback techniques to calculate the output impedance,
I have to calculate zoA,

and Zof= zoA(1 + A*f)

A=Io/Vi=gmAd/1+gm*RE+RE/ro =2.47
because Io= gm(AdVi - IoRE) -IoRE/ro

zoA= V2/I2= RE + gmREro + ro = 6.064Mohmios

f=RE

Zof= 60Gohmios