From buffer to amp stage mod. Its possibile to do?

Audioguru again

Joined Oct 21, 2019
3,545
Your 2N3906 PNP transistors are upside down. Their emitters should go towards positive voltages.
The input impedance is fairly low at 5.1k ohms.
The circuit cannot have any gain.
 

Audioguru again

Joined Oct 21, 2019
3,545
The simulation does not know when a transistor is upside down or when a transistor is overloaded.
In a real circuit the reverse-biased emitter-base junctions will have avalanche breakdown.
My simulation works the same when the PNP transistors are backwards or forwards and at your very high radio frequency of 100MHz!

Your new circuit is missing the power supplies.
 

Thread Starter

Zeno53

Joined Sep 23, 2020
89
In the image the power supplies are missing but in the simulation there are. At the output it generates the same input voltage and in fact the two signals are superimposed. As it is being a buffer it should not amplify and what I asked is if it is possible to modify it to have an output gain. Otherwise, I don't know what to do to solve the problem, but these transistors have no problem working at 100MHz.

merge.PNG
 

Papabravo

Joined Feb 24, 2006
16,832
As you have it drawn there is very little possibility of voltage gain. There can be current gain and power gain by going with a class AB configuration like the following:
 

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Thread Starter

Zeno53

Joined Sep 23, 2020
89
I see now that PNP PNP transistor work up to 4Mhz. Its is possible to use another transistor like these above?
 

DickCappels

Joined Aug 21, 2008
7,743
It is a pretty straight-forward emitter driven buffer. There is no easy way to modify it to get gain without major rework. I suggest starting with a different amplifier.
 

Papabravo

Joined Feb 24, 2006
16,832
Hello @Papabravo

so this because its a buffer and not "moddable" right? Do you have also a class A stage?

Thank you
I don't understand exactly what "not moddable" means. If it means "Can I modify a class B emitter follower stage to get voltage gain", then the answer is no. The second question is: what kind of gain can I get, and the answer is you can get current gain and you can get power gain.

My schematic is called a class AB because Q3 is a class A common emitter stage.
 

Thread Starter

Zeno53

Joined Sep 23, 2020
89
I mean not electrically editable. I was looking at the circuit you attached to me. I don't have the library to test those two transistors. I was looking for the spice files but can't find them. Do you know if there are any equivalents on the standard ltspice library? I took a look at HERE but of I have not found true equivalents.
 

Papabravo

Joined Feb 24, 2006
16,832
I mean not electrically editable. I was looking at the circuit you attached to me. I don't have the library to test those two transistors. I was looking for the spice files but can't find them. Do you know if there are any equivalents on the standard ltspice library? I took a look at HERE but of I have not found true equivalents.
I went to the ON Semiconductor website, found the models on the same page as the datasheet and added them to the standard library.
You can use any suitable power transistors that you have models for. If you actually want to build what you simulate then you also have to have models for parts you can buy. I don't know if you have noticed but through hole transistor parts are starting to get scarce.

The file below is a family of four
 

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Thread Starter

Zeno53

Joined Sep 23, 2020
89
@Audioguru again no need buffer. I asked if that buffer was transformable into a gain stage.
@Papabravo With the placement of the transistor I solved. Obviously the default position is not the one in your image. Regarding the spice on ON I have seen them now but are they compatible with ltspice? Do the ones you put in (thanks) also need to be renamed to something? I have not yet addressed the creation of the component on ltspice. Other times I found myself wanting to simulate pspice components on ltspice but they are not directly compatible and I am unfortunately stuck. I repeat the question. Is it possible to use that circuit with 3904 3906? I need to go up in frequency and the 58xx series doesn't.
 

AnalogKid

Joined Aug 1, 2013
9,361
I believe there is a problem with the PNP symbol being upside down.
The schematic you posted in post #1, and the EDN schematic you linked to in post #3, are not the same. The EDN schematic is correct. And, that circuit cannot be modified to have gain greater than 1 without significant changes.

ak
 
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