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motbuddy 28th September 2011 09:33 AM

Does this circuit consider a direct coupled amplifier?
1 Attachment(s)
Hi all,

Does the figure below consider a direct coupled amplifier? when C1 and C3 is connected to the circuit.
Attachment 242190

Or i should ask, does the output of a direct coupled amplifier is an ac sine wave? because seems like dc amplifier take out all the coupling capacitor

thank you

DF96 28th September 2011 10:48 AM

Q2 is directly coupled to Q2. R3 and R4 can be removed, as they have no function. The output from a directly coupled amplifier can include AC - it depends on what the input signal is.

Are you an EE student?

AndrewT 28th September 2011 10:59 AM

the two stages are direct coupled.
The input and the output are AC coupled.

vacuphile 28th September 2011 01:11 PM

Motbuddy, even if you leave out c1 and c2, I don't think you could call this a direct coupled amplifier, since it would not work. The reason is that it has a single supply voltage and no independent ground.

In other words, it cannot do DC at 0 volts, and it only does AC clipped in less than half. Plus that your input device would see a high DC voltage offset. If that input device is direct coupled with a servo to keep a low DC offset, things would start fighting each other.


motbuddy 28th September 2011 02:24 PM

hi DF96, yeap i'm an EEE student.

hi vacuphile, when i add in c1 and c3, i'm able to see an amplified output signal. but i remove both of it i not able to get any output signal.

vacuphile 28th September 2011 03:11 PM

Motbuddy, that makes sense. Without c1, the input node is somewhere between ground and Vs. Please measure with volt meter. It will be way above the signal, if that is referenced to ground. You are lucky not to have damaged your input device (source).

At the output node you will also detect DC, at a voltage somewhere between ground and Vs, without the coupling cap.

In this setup, it just won't work as a direct coupled amplifier for the reason I mentioned before. Those coupling caps are there for a good reason, and now you know why: in order to isolate DC.


wwenze 28th September 2011 03:15 PM

Such an amp (running off a single supply) need C1 and C3 to work. Collector should be biased at 1/2 Vcc. Output signal without C3 is a sine wave with DC offset, C3 removes the DC component.

The high DC might be clipping your measuring device so you are unable to see the sine part.

And one more thing... without C3, the load, if too low, can affect the biasing too.

motbuddy 29th September 2011 02:17 AM

1 Attachment(s)
so if i was given a task to construct and explain a direct coupled amplifier then this circuit works. am i right?

and the post u mentioned which using more than one supply for the circuit, is it this is what u mean? (i google it)
Attachment 242281

thank you.

AndrewT 29th September 2011 09:20 AM

why are you choosing to use three transistors?
What has the task specified?
Use one transistor as your DC coupled amplifier, if that meets the task specification.

DF96 29th September 2011 09:29 AM

If you want us to help you with your homework it would help if you told us the actual question you are trying to answer. Also, what you/your teacher means by "direct-coupled", as it is a phrase not used very often these days. 40 years ago when almost all audio circuits used AC coupling throughout, a direct-coupled circuit was new and different. Nowadays almost all solid-state audio circuits are direct-coupled internally so the term is not used so much. What would be unusual would be a solid-state circuit with AC coupling everywhere. It is possible that your teacher or the textbook he is working from is based on practice 40 years ago!

Finally, what investigations have you done (apart from posting here) to learn what is meant, and how to answer it? For example, do you understand why it is not possible to create a direct-coupled circuit simply by removing the coupling caps from an AC coupled circuit? Your first post suggests not.

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