Discussion in 'Homework Help' started by Teknolog, Nov 21, 2013.

1. ### Teknolog Thread Starter Member

Sep 1, 2013
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If I want to cascade a voltage amplifier, a transconductance amplifier and a transresistance amplifier, how should they be connected?

Would this connection work for example? The Load is the lower resistor to the far right.

Last edited: Nov 21, 2013

Apr 5, 2008
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Hello,

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Bertus

3. ### Teknolog Thread Starter Member

Sep 1, 2013
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Ok thanks, trying again:

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4. ### WBahn Moderator

Mar 31, 2012
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This won't work because the load is in series with the feedback element of the transconductance amplifier.

5. ### LvW Well-Known Member

Jun 13, 2013
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* The circuit has no common ground
* The second amplifier has no negative feedback
* The third amplifier has positive instead of negative feedback.

More than that - don`t forget that a transconductance amplifier has a current output (rather than voltage).

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6. ### WBahn Moderator

Mar 31, 2012
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Oh, the second amplifier has negative feedback, and it is basically a transconductance amplifier in that the input voltage will determine the current flowing in the feedback resistor and, in this case, the final load. But the output current from the second amp will not be the same as the current in the feedback resistors, so the output of the second amp can't be considered a transconductance relationship to the input.

Let's say the input signal is 0.1V. The output of the first stage would be 1.1V. This would result in 1.1mA in both of the second stages feedback resistors which would result in 2.2V at the inverting input of the final amp. This same 1.1mA would flow in the load resulting in 3.3V at the output of the final amp. With 2.2V at the inverting input and 3.3V at the output, the current through the resistor going to the non-inverting input of the final opamp would be 1.1mA. This current would flow back and INTO the output of the second amp, so even though the magnitude of the current flowing out of the nominal transconductance amp is coincidentally the same as the current "returning" through the feedback network -- coincidental because change any of several resistors and that they will no longer be equal even in magnitude -- but it is in the opposite direction.

I think all of the feedback connections are in the right direction, but I'm not sure that there isn't a stability issue between the final two stages.

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7. ### Teknolog Thread Starter Member

Sep 1, 2013
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Allright, I tried to fix it like you suggested and made a new circuit. Would it work now?

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8. ### WBahn Moderator

Mar 31, 2012
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Notice that now your second stage is simply a voltage amplifier with a gain of 2. It has the same topology as the first state, right? The third state is now a comparator with positive feedback.

9. ### Teknolog Thread Starter Member

Sep 1, 2013
31
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Hm, right. I think I need some help designing this circuit. How can the transconductance amplifier at the same time give negative feedback and also have it's output current connected to the next amplifier?

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
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I think second stage should look like this. The third stage is the load resistor of the second stage.

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11. ### LvW Well-Known Member

Jun 13, 2013
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1.) Is the second amplifier an OTA? In this case you should use a corresponding symbol
2.) What is the reason for positive feedback of the last stage? It will not work as expected.
3.) Still the common ground is missing (one can imagine where it is).

12. ### Teknolog Thread Starter Member

Sep 1, 2013
31
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shteii01: I don't think that works, looks to me as if you've switched places of the load (third op circuit) and the resistor in the second amplifier. Also I'm thinking the output of the second amp should go into the - input of the third?

LvW: I want to use the op amp "TL071" in all three stages.

I still haven't figured out how to connect them. Any help with circuit design for the second and third amps is welcome.

13. ### LvW Well-Known Member

Jun 13, 2013
769
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No OTA anymore? And what is the purpose of the whole amplifier chain?
You want help for the 2nd and 3rd amplifier - but we do not know what your intention is.

14. ### Teknolog Thread Starter Member

Sep 1, 2013
31
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The purpose is just to construct a working amplifier chain. Then I'm going to test how the the amplification and phase changes for higher frequencies. But first I need to construct a cascade chain that works like intended (i.e. One voltage amp, one transcond. amp and one transres. amp).

15. ### Teknolog Thread Starter Member

Sep 1, 2013
31
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Would this connection work? I made it so that there's negative feedback in the second op amp but it comes all the way from the third op amp. Also the third op doesn't have common ground. I don't know how to connect it otherwise though?

Alternatively, it would be

Here they have common ground but the second op amp doesn't have negative feedback.

Doesn't anyone know how to do this???

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16. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Ok. Check this out http://www.wisc-online.com/Objects/ViewObject.aspx?ID=SSE3303

Sep 1, 2013
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18. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Ok. I think I see what I did wrong.

Last edited: Nov 23, 2013
19. ### WBahn Moderator

Mar 31, 2012
23,572
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So, what is the voltage at the inverting input of the second opamp? Is it the same as the non-inverting input of the second opamp, or of the third?

@OP: Draw each stage differently, clearly identifying the input and output for the stage. Then it should become fairly apparent how they can be interconnected or whether there are issues with the connections that would be needed.

20. ### Teknolog Thread Starter Member

Sep 1, 2013
31
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I already knew what was said there, and that is still not what I'm asking for, so why are you posting useless links?