Amp Circuit Explanation

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Sambuchi
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Amp Circuit Explanation

Post by Sambuchi » Wed Jul 15, 2009 5:18 am

Hello all, I have a circuit that I found on the net and was hoping to get some feedback on it. It is an amplification circuit that can be turned on/off. DET_CH0 is the signal input with 50 impedance. PWR_CH0 turns power on off to the circuit and DET_PULSE_CH0 is the out. Looking at the circuit you can notice that they repeat some stages of the circuit over and over. I was hoping to identify what type of circuit these are. I can run the PSpice simulation to get great results but I dont know the verbiage.

The circuit can be found here.
www.sambuchi.net/amp.JPG

Thanks again.

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MrAl
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Re: Amp Circuit Explanation

Post by MrAl » Wed Jul 15, 2009 2:54 pm

Hi there,


Wow, that's quite a big circuit there for an amp. Can i ask what you want to detect with this?
UFO signals from deep space? :smile:

Really though, what is this going to be used for?
I ask because there may be simplifications.
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Sambuchi
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Re: Amp Circuit Explanation

Post by Sambuchi » Thu Jul 16, 2009 11:51 am

This 3 stage amp has a 1000 x amplification to a IR signal coming in.

I'm guessing that the first stage is filter out high frequencies and at the end of each stage has a comparator.

That is the best I can think of. I need something that can do this. Hopefully with today IC's this circuit can be simplified! :grin:

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MrAl
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Re: Amp Circuit Explanation

Post by MrAl » Thu Jul 16, 2009 4:08 pm

Hi again,


Well, that explanation helps but still doesnt spill all the beans. We need to know more about
what exactly this is going to be used for. Im sure you can get a lot of gain out of two op amp
stages, but what kind of op amps will depend on what bandwidth is needed and maybe noise.
It helps to decide these things knowing what it will be used for in real life.
LEDs vs Bulbs, LEDs are winning.

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Bob Scott
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Re: Amp Circuit Explanation

Post by Bob Scott » Fri Jul 17, 2009 7:01 pm

The way I see this circuit:

The power on signal goes to the gate of Q10. Q10 is an inverting voltage amp driving Q9, the power switch. From the drain of Q9, the power goes to a 2 stage noise decoupling network before it goes through L19 to supply power to the device "DET module". It looks like this DET module works like a condenser mic, where the module power input and signal out are on the same wire.

DET signal coming in encounters a frequency trap composed of C106, L20, and R129. It looks like a narrow band frequency trap tuned to <Bob brings up Winders calculator><f = 1/(2*Pi*SQRT(L*C))>Oops, the value of L20 is not given. R129 sets the damping of the series tuned circuit C106 and L20, so we could calculate the Q if this filter if we either knew the value of L20 or knew the frequency it was tuned to. But we don't.

Signal comes in through L19 and DC blocking cap C109 to the base of Q15. Q15 is an inverting voltage amplifier, supplying signal to Q12, a simple emitter follower current amplifier. Gain of this 2 stage amp is set by feedback network that includes R130, C110 and the output impedance of the DET module. The gain is calculated by dividing the value of R130 by the DET module output impedance. I don't have that information. Gain rolls off as a low pass filter. The corner frequency where the rolloff starts is where the capacitive reactance of C110 is equal to the value of R130, 2.2K Ohms. We have that information. f = 1/2*Pi*C*Xc where Xc = 2.2K. I get 10.6 MHz.

[EDIT: I see I missed your statement that DET is 50 Ohms, so the gain of the 2 transistor stage is 2,200/50 = 44. In Db, that's 33Db.]

I don't know if there is a technical name for the way Q12 and Q15 are set up. A lot of old audio preamps used this configuration.

I could go on, but do you need information in this much detail? More detail? I could work out the DC voltage present on the transistor terminals.
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Re: Amp Circuit Explanation

Post by Robert Reed » Fri Jul 17, 2009 8:16 pm

I have to agree with Bob for the most part. This is one weird circuit and knowing what signal content you have and just what you want to do with it would go a long way (not to mention some values for the inductors). For starters, and without calculating DC biases at every stage It looks like all stages are at least conducting somewhere from Class A to AB. So there appears to be no detection going on here as in the normal sense (or does DET-CHO already imply a detected signal ?). The source device may have a transistor with an uncommitted collector hence the highly filtered DC feeding the signal input line with R120 acting as it's load resistor. The end circuit is what I cannot follow as Q20 & Q19 form a differential amp which would either have a differential signal fed into it (not in your case) or Q19's base held at signal common. The signal transfer would normally be emitter coupled, but not possibble here due to C107 & C108 preventing dynamic transfer for all but very low freqencies. Only static transfer of DC here. Yet the signal out put is still shown coming off of the tapped Q19 load resistor to the last amplifier stage. I could have missed something along the way here by not thoroughly taking exact stage biases into consideration but am still at a loss on the diff amp. (Q19, Q20).

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Sambuchi
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Re: Amp Circuit Explanation

Post by Sambuchi » Tue Jul 21, 2009 7:23 am

I forgot to thank everyone for their input!

Thank you very much! I now have a better understanding what is happening here.

My input conditions are:
The pulse width is 50ns and periodic 90 ms.
The min signal pwr that needs to be detected is 560nA

Again your input is greatly appreciated.

Tony

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