How to calculate the value of Zin and Zout???

[samy555]

Can you please, help me in the calculation of input and output impedance of the following circuit:

This stage operates as the oscillator in a FM transmitter, and I want to put a matching Pi network between it and the 50 Ω antenna, so I need to calculate its input and output impedances.
Thanks

 

[chuckey]

The Zin is 47K || with Hfe X 470 ohms. Hfe = 150?, 150 X 470 = 60K, in parallel with 47 ~ 25 K.
Zout is difficult, its theoreticaly the impedance of the tuned circuit ( L/CRloss) in parallel with the output impedance of the transistor. The trouble is that as you match the 50 ohms to the transistor, you loose gain so you can have a perfect match but no oscillations. I would just use a 1/2 turn link of thick wire wound around the non-collector end of the coil.
Frank

 

[samy555]

Thank you Frank

Zout is difficult, its theoreticaly the impedance of the tuned circuit ( L/CRloss) in parallel with the output impedance of the transistor.

(1) Suppose that L= 65nH, c= 47pF and Rloss measured = 0.01 ohm, then the impedance of the tuned circuit = (XL+Rloss)// XC?? is it true????
(2) Are you mean by "the output impedance of the transistor" is the (1/hoe)??

The trouble is that as you match the 50 ohms to the transistor, you loose gain so you can have a perfect match but no oscillations. I would just use a 1/2 turn link of thick wire wound around the non-collector end of the coil.

What if I use a transformer like this:
**broken link removed**

Last question
If I implement the circuit using the multisim 10, Is there a way to measure the output impedance using the program?

thank you very much

 

[chuckey]

In the above circuit diagram, the aerial is shown as 30 cms long. At 27 MHZ, a wavelength is 11m, so a quarter wavelength (=75 ohms!!) is 11/4m ~ 2.75m. So a 30 cm long aerial is VERY short compared to a quarter wave length so its impedance is likely to be more like 10 ohms + a capacitive reactance of -j 100 (~6 pF). The 8 Turn coil tunes out the capacitive reqactance so the next 100 pF sees only the 10 ohm impedance. The PI netwok then transforms this up to an impedance of 10 X 625/225 = ~30 ohms, the link then transforms this up to an impedance of 16:1 = 480 Ohms.
1. R loss = .01 ohms XL @27 MHZ = 12 ohms, therefore QL = XL/r = 12/.01 = 1200 much too high, should be 50 -100.
2. Yes
Put a low value resistor as a load (say 20 ohms), let the measured voltage be V20, repeat using a different value say 30 ohms, V30.
So the current is the first case is (V20/20) = I20, in the second,(V30/30) = I30. So the change of voltage is V20 - V30, the change in current is I20 - I30. So the output impedance is (V20 -V30)/(I20 - I30).
Frank

 

[samy555]

Is the following true?
1- The 2nd transistor acts as an oscillator, so the 1n cap made it a common base?
2- Since the 1st stage is a voice preamplifier so it supply a low frequency signal to the 2nd, that is the 1st see the 2nd as a common emitter?

thanks

 

[neddie]

You have not given a frequency at which you want the output and input impedances. Running the cct through LTspice using the network function and a inductance of 300nh shows
input impedace of only 1.5ohms. (100Mhz) That is due to the 1n capacitor at the input , which is needed for the cct to oscillate. Without the cap it is 140ohms , angle -67deg , give or take. Output is 32 ohms angle - 85deg.
I'm actually using a bc337 , but the values will be in the ballpark. Trying to match an antenna to an oscillator could be problematic. If your match is too good you will kill the osccillations.

 

[samy555]

Where is the network function in the LTspice??

you may calculate freq from the tank parameters, assume it 100 MHz

the signal input to the oscillator sees a common emitter not CB because it is a low freq voice signal

I dont want match anything,,,, I want to understand some way to measure or calculate the output impedance for the oscillator

thanks alot

 

[neddie]

Look in the manual for .net commands.
Also look on the internet for this PDF ( LTspice_4_e2.pdf) . Great intro , and more advanced stuff for LTSpice.
I'm not going to attach as it's too big.

 

[samy555]

Look in the manual for .net commands.
Also look on the internet for this PDF ( LTspice_4_e2.pdf) . Great intro , and more advanced stuff for LTSpice.
I'm not going to attach as it's too big.

You really put me on the right path beginning, but you have to supplement with me to get to the result.
I read the ( LTspice_4_e2.pdf) carefully and succeeded in getting both S11 and S22 graphs
both S11 and S22 are gotten in magnitude and phase, and I can convert them from that polar case to the rectangular (real &imag), my qustion is what to do after that, i.e how to calculate the input and output impedance?

One last point remains I hope to receive more explanation around, that is about: AC 1 0 and (Rser=50)
In my circuit, the oscillator feeds from a precedent stage (the voice preamplifier), so I assumed that it is a 100mV 10K ohm source:

Do you have any comment on it?

thanks alot

 

[FvM]

The S11 and Zin point is meaningless for the present circuit, at least regarding antenna matching, because it's an oscillator and has a RF-wise input short.

 

[samy555]

The S11 and Zin point is meaningless for the present circuit, at least regarding antenna matching, because it's an oscillator and has a RF-wise input short.

I do not understand you

 

[FvM]

To answer with a question. What do you expect to see from S11 respectively Zin? How is it related to available antenna power or antenna matching?

 

[samy555]

Can you tell me how these people design of such those circuits? They definitely relying on something.

 

[neddie]

In the list of traces you can add to the plot is Zout. Look through carefully. As mentioned above Zin / S11 is not really of any use here.

 

[samy555]

In the list of traces you can add to the plot is Zout. Look through carefully. As mentioned above Zin / S11 is not really of any use here.

There are no Zout but Zout(V2)
Are they the same thing?

 

[FvM]

The network parameters are referring to the ports defined in the .net statement. Output is to R2, independent of the V2 connection.

 

[Majid Rafei]

How can I save the results of the .Net analysis (in LTspice) into a text file directly by executing scad3.exe from the command window?