Amplifying current of an oscillator circuit

Thread Starter

DD Ki Vines

Joined Apr 20, 2019
21
Today I have made a RC phase shift oscillator producing 90KHz of sine wave with a 6V dc offset.Now I want to amplify its current upto 100mA amplifying the overall power of the signal, I have tried amplifying using 2n3904 and as well as 2n6292 but none of them worked I hardly got 14mA max using those,first I tried without using any base resistance(fig1) then I added one 3.3K resistor to the base(fig2),detailed schematics of the circuit is attached below please someone help me fix it or suggest me a better idea of doing this,thank youIMG_20210806_191558.jpg
 
Last edited:

Papabravo

Joined Feb 24, 2006
17,030
It might help if you defined your actual requirements, and then selected a configuration capable of achieving that goal. The 2N3904 is hardly what I would call power transistor, but it can still provide considerable current to a load if used correctly. The 2N6292 is a legitimate power transistor but requires a suitable driving circuit to produce it's rated output current. The configuration you have drawn, with and without a base resistor, is called a common collector or emitter follower. It is not clear from your schematic how you are connecting the oscillator to the transistor. EDIT: I just found the Vout notation on the output of the amplifier. Why did you rotate your diagram 90° and make the writing too small to read accurately?

In order to function correctly the transistor must have an appropriate DC bias circuit as well as a connection to an AC driving source that will not change the characteristics of the oscillator by presenting an additional load. You may need to buffer the output of the oscillator before connecting it to the transistor.
 

Thread Starter

DD Ki Vines

Joined Apr 20, 2019
21
It might help if you defined your actual requirements, and then selected a configuration capable of achieving that goal. The 2N3904 is hardly what I would call power transistor, but it can still provide considerable current to a load if used correctly. The 2N6292 is a legitimate power transistor but requires a suitable driving circuit to produce it's rated output current. The configuration you have drawn, with and without a base resistor, is called a common collector or emitter follower. It is not clear from your schematic how you are connecting the oscillator to the transistor. EDIT: I just found the Vout notation on the output of the amplifier. Why did you rotate your diagram 90° and make the writing too small to read accurately?

In order to function correctly the transistor must have an appropriate DC bias circuit as well as a connection to an AC driving source that will not change the characteristics of the oscillator by presenting an additional load. You may need to buffer the output of the oscillator before connecting it to the transistor.
I actually want to design an AM modulator(Switching modulator) and this oscillator produces the carrier wave.I am planning to use a 1N4148 diode as a switching device,for proper modulation the diode needs to be in linear region that means high current that is why I need about 100mA of current
 

Papabravo

Joined Feb 24, 2006
17,030
I actually want to design an AM modulator(Switching modulator) and this oscillator produces the carrier wave.I am planning to use a 1N4148 diode as a switching device,for proper modulation the diode needs to be in linear region that means high current that is why I need about 100mA of current
So of all the possible transistor configurations possible, which one do you think is most appropriate and why? For each configuration do you know haw to calculate the voltage gain, the current gain, and the power gain. If so great, and if not, then why not?
Next question is how do you properly provide DC bias for a transistor amplifier?
Last question is how do you AC couple the signal from the oscillator to the amplifier?
 

Thread Starter

DD Ki Vines

Joined Apr 20, 2019
21
So of all the possible transistor configurations possible, which one do you think is most appropriate and why? For each configuration do you know haw to calculate the voltage gain, the current gain, and the power gain. If so great, and if not, then why not?
Next question is how do you properly provide DC bias for a transistor amplifier?
Last question is how do you AC couple the signal from the oscillator to the amplifier?

I have already amplified the voltage so I don't need to amplify the voltage anymore thats why I chose the emitter follower(Unity Voltage gain),and I put the ac voltage with 6V dc offset at the input to keep Vce at the middle of the load line so that I get the full swing and I definately got that but I am not being able to amplify the current it looks like Base emitter junction of the transistor is loading the input signal. I havent mentioned this in the thread but I have actually used an unity gain buffer at the output of the oscillator first, and output of that buffer is connected to the input of the transistor but it isn't working, I know i/p impedance of a transistor is Rpi=beta/gm=(beta*VT)/Ic,according to the datasheet of 2n6292 beta can vary from 30-150(for Ic=2A and Vce=4V) so I have taken beta as 50 and VT=26mv and Ic=100 mA so Rpi becomes almost 13Ohm that's why I added the 3.3K ohm in series with the base but then also it didnt work,can you please help me solve this problem
 

Papabravo

Joined Feb 24, 2006
17,030
You did not mention that you AC coupled the signal from the unity gain buffer to the transistor and I see no evidence, in the schematic you have provided so far, that you have constructed a proper DC biasing arrangement for the emitter follower. You need to update your schematic including the opamp part number and the power supply used for the opamp.
 

Thread Starter

DD Ki Vines

Joined Apr 20, 2019
21
You did not mention that you AC coupled the signal from the unity gain buffer to the transistor and I see no evidence, in the schematic you have provided so far, that you have constructed a proper DC biasing arrangement for the emitter follower. You need to update your schematic including the opamp part number and the power supply used for the opamp.

This is the whole schematics
edit:Op amp used TL-084 and transistor used 2n6292 you need not to worry about the power supply it can provide upto 24V 2A
 

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Papabravo

Joined Feb 24, 2006
17,030
This is the whole schematics
edit:Op amp used TL-084 and transistor used 2n6292 you need not to worry about the power supply it can provide upto 24V 2A
The answer I was looking for was single supply or bipolar supply. Single it is.
Still no AC coupling and no stable bias supply.
 

Thread Starter

DD Ki Vines

Joined Apr 20, 2019
21
The answer I was looking for was single supply or bipolar supply. Single it is.
Still no AC coupling and no stable bias supply.
I can't put a capacitor at the output it will block the dc which I need to supply to the input of the transistor to keep the AC voltage above the ground level otherwise it will be clipped and I will be forced to use another PNP transistor in a push pull configuration which will need a bipolar power supply which I don't want to use and what do you mean by no stable bias supply please explain
 

Papabravo

Joined Feb 24, 2006
17,030
I can't put a capacitor at the output it will block the dc which I need to supply to the input of the transistor to keep the AC voltage above the ground level otherwise it will be clipped and I will be forced to use another PNP transistor in a push pull configuration which will need a bipolar power supply which I don't want to use and what do you mean by no stable bias supply please explain
You do it like this: R1 & R3 supply the stable bias of 6 VDC, required by Q1. The RMS current through the emitter is in excess of 100 mA(RMS). The voltage source V2 represents your TL084 unity gain buffer which should be more than capable of supplying the required 1 mA drive current, and is AC coupled to the Emitter Follower You did not specify the frequency you are interested in so I don't know if that will be a problem. Is this an optimal solution? No, I just threw it together. An optimal solution would actually take more than an afternoon.
1628268508185.png
 

AnalogKid

Joined Aug 1, 2013
9,408
A problem with both of the single-transistor circuits shown is that an emitter follower has an asymmetrical output impedance. The positive half-cycles have a very low output impedance (the conductance of the transistor), but the only thing pulling the output down through the negative half cycles is the load impedance. Consider a complimentary output stage. There are many app notes on how to add a current buffer to an opamp.

ak
 

Papabravo

Joined Feb 24, 2006
17,030
A problem with both of the single-transistor circuits shown is that an emitter follower has an asymmetrical output impedance. The positive half-cycles have a very low output impedance (the conductance of the transistor), but the only thing pulling the output down through the negative half cycles is the load impedance. Consider a complimentary output stage. There are many app notes on how to add a current buffer to an opamp.

ak
What you say is true, but the TS needs to take some intermediate steps to get there. Not understanding the concept of a "stable bias supply" is a huge impediment. Also the concept that amplification requires DC coupling is another impediment. We can fix all of these problems, just not quickly. It will take some time and patience.

BTW - Resistor R2 should be a 1 watt resistor, if you are going to test this on something you can afford to lose, since it will be dissipating 500 mW and some margin is desirable.
 

Audioguru again

Joined Oct 21, 2019
3,675
The simulation software does not show the smoke produced by the overloaded little 2N3904 transistor. It dissipates 6.8V x 100mA= 680mW but its maximum allowed dissipation is only 625mW.
 

Papabravo

Joined Feb 24, 2006
17,030
When You guys get done with the Discreet Parts Lesson.
This Op-Amp will do the job perfectly,
and includes "3-State-Outputs" which may be a bonus for your Modulation-Scheme.
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I missed the part where they gave you leave to be snarky and ah....condescending...yeah, that's the ticket.
I thought we were trying to help the TS come to his own conclusions, instead of telling him what to drink and where to find it.
 

LowQCab

Joined Nov 6, 2012
1,189
No snarkyness intended, sorry if I stepped on your toes.

I've just noticed that,
(especially with people who can write Code, and assume that every Circuit needs a processor),
tend to get stuck on the most very basic "sticks and rocks" of I/O methods.

So, I'm simply trying to broaden their options.
"There's more than one way to skin a Cat" and I'm always looking for a better way.
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Papabravo

Joined Feb 24, 2006
17,030
No snarkyness intended, sorry if I stepped on your toes.

I've just noticed that,
(especially with people who can write Code, and assume that every Circuit needs a processor),
tend to get stuck on the most very basic "sticks and rocks" of I/O methods.

So, I'm simply trying to broaden their options.
"There's more than one way to skin a Cat" and I'm always looking for a better way.
.
.
.
No problem if we are trying to engineer a solution. Sometimes the goal is to lead the TS in that direction when the original concept fails to provide the desired results.
 
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