Voltage Controlled Attenuator (VCA): Electronic Gain AC Analysis
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- Опубліковано 24 сер 2024
- AC signal analysis and deriving the electronic gain control formula for this Voltage-Controlled Attenuator (VCA) are presented in this part-2 video of an OpAmp-BJT implementation of VCA circuit. One important application of this VCA circuit is in audio signal processing and Automatic Gain Control (AGC) for audio VCA. In this implementation, the electronic gain/attenuation control circuit is designed with three Operational amplifiers and six NPN Bipolar Junction transistors. In previous Part1 video • VCA Electronic Gain Co... , an intuitive circuit overview and DC bias analysis of this electronically-controlled gain/attenuator circuit are discussed. The follow-up Part three presents a variant of this circuit that then enables positive voltage control instead of the negative control voltage (Vc) that is needed in the VCA circuit analyzed in parts one and two. This three-part VCA Electronic Gain Control circuit design is posted in the Analog Circuits video playlist • Electrical Engineering... . Note that the presented Voltage-Controlled Attenuator is an example of electronic gain control circuit that is implemented with three operational amplifiers and six NPN bipolar junction transistors. Four matched BJT transistors are used to construct two differential BJT stages. These matched NPN transistors should be in the same IC package and on the same substrate so that they share the same PN junction process and thermal properties and hence have the same PN Junction saturation current and thermal voltage (VT). The bottom NPN BJT transistors provide the DC bias current for the two differential BJT pairs. When applied control voltage Vc is zero, circuit is symmetric (matching transistors) resulting in half of the bias current passing through left and right branches of the differential pair. If a negative control voltage is applied, it steers current away from one branch to the branch of the differential BJT stage. The complete DC bias analysis presented in this video intuitively explains how the circuit is biased and also computes the DC currents and voltages of the nodes in this circuit. It is show that output DC voltage is zero.
AC signal analysis and deriving the Electronic Gain Control formula for this Voltage-Controlled Attenuator (VCA) are presented. Please see the first video ua-cam.com/video/cFzYZsPEtP0/v-deo.html for circuit overview and DC bias analysis of this electronically-controlled gain/attenuator circuit. The follow-up Part three presents a variant of this circuit that then enables positive voltage control instead of the negative control voltage (Vc). This three-part VCA Electronic Gain Control circuit design is posted in the Analog Circuits video playlist ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html.
And for related analog circuit examples see:
Op Amp Amplifier with Electronic Gain Control ua-cam.com/video/NoNgQpbj77Y/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
Analog Computer Vector Sum Calculator Design with Op Amp, BJT Transistors ua-cam.com/video/PIAsa0QNVns/v-deo.html
I hope these examples of Electronic Gain, Attenuation Control Circuits are useful and interesting. 🙏
Could you explain what advantages and disadvantages this type of circuit has in comparison to the so-called 'Blackmer Gain Cell'?
Thanks for your good question. There are multiple ways to design analog multiplier including the Blackmer Gain Cell and also the Gilbert Cell architectures that. As another implementation please see Analog Multiplier Circuit ua-cam.com/video/VP53A2zpVMQ/v-deo.html . I would say input op amps in this circuit provide a better isolation for input signal and output Op Amp minimizes the impact of output load on the operation of this circuit. Given that Audio Op Amps are relatively cheap, I would say it is a minimal addition to BOM cost of design in exchange for the aforementioned benefits. Note that it is also possible to modify and enhance Blackmer Gain Cell or the Gilbert Cell. In summary, the fundamental idea behind all these techniques is effectively similar since they all rely on the BJT Shockley PN junction logarithmic V-I equation of Bipolar junction transistor to implement analog multiplication or signal modulation. I hope this clarification is helpful.
This video series is fantastic, thank you so much for doing these. But I have to ask, where is part 3?? I'd love to see the variations but can't find it anywhere!
You are very welcome. Glad that my channel and videos are useful. Part three of this Voltage Control Attenuator (VCA) Circuit will be a minor extension of Part two that I will post hopefully soon. Here are few relevant videos:
Op Amp Amplifier with Electronic Gain Control ua-cam.com/video/NoNgQpbj77Y/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
Analog Computer Vector Sum Calculator Design with Op Amp, BJT Transistors ua-cam.com/video/PIAsa0QNVns/v-deo.html
I hope these electronic amplifier videos interesting as well. Thanks again for your interest in my channel. 🙋
@traviskirbycompositionscor4626 Thanks again. Just posted a related video ua-cam.com/video/sZTjirsZFZE/v-deo.html that hopefully is interesting as well.
@@STEMprof Thank you for responding! I was wondering, what are the modifications required to make this circuit functional using a positive voltage for Vc? If you're able to state them in such a limited format as a comment.
@@STEMprof This is awesome! Loved the video. What type of modulation applications would this modulator circuit be best used for?
@@traviskirbycompositionscor4626 Thank you! Glad that you liked this Analog Modulation Video. An example for instance is in analog Audio signal processing whenever one needs to combine two portions of music before modulating the combined signal. More Amplifier videos are posted in ua-cam.com/play/PLrwXF7N522y7Ut9bm8TXAOhIWqL__FGlj.html Playlist.
Great Video prof, many Thanks! Please Can you explain this AGC without opamp and built with BJT only? I mean of a circuit like an old AM radio transistors, thanks and best regards
You're welcome! Thanks for watching and your good question. You are probably referring to designing analog AGC using a core similar to the BJT Gilbert Cell. I'll try to post a video about that. For a related example please see Analog Multiplier Circuit ua-cam.com/video/VP53A2zpVMQ/v-deo.html . Please note that the fundamental concept behind all these techniques is effectively similar since they all rely on the Shockley PN junction logarithmic V-I equation to implement analog multiplication or signal modulation. I hope this clarification is helpful.
@@STEMprofI will watch the link, thank you!
You're welcome.
My brain 🤯
Thanks for watching.🙂 Was this part2 more difficult than Part1? Is this circuit interesting? I tried my best to explain the AC analysis and proof of gain formula (Part2 Video) for this VCA Electronic Gain Control circuit. I hope it is helpful at least to some extent. 🙋♂️
@@STEMprof thank you yes i think i understood it somewhat. I dont know much electronics since i just started learning this year so it is difficult for me.
@@rudygomez6996 You are doing very well studying these more advanced circuits if you just recently started studying Electronic circuits. Well done! 👍
Does this circuit have a specific name?
Sure, as the video title indicates, this is a VCA or Voltage-Controlled Attenuator Circuit which is a variant of the Electronic Gain Control circuits. For related analog circuit examples see:
Op Amp Amplifier with Electronic Gain Control ua-cam.com/video/NoNgQpbj77Y/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
Analog Computer Vector Sum Calculator Design with Op Amp, BJT Transistors ua-cam.com/video/PIAsa0QNVns/v-deo.html
I hope these examples are interesting as well.
@STEMprof What I meant to ask was: Is there a more specific name for this circuit? If I google voltage-controlled attenuator then I will find a lot of circuits that achieve voltage controlled attenuation, but not this specific circuit.
@@commanderkoen That's a good question. This is an example of the large category of electronically-controlled amplifiers/attenuators. There are many ways to design adjustable amplifiers. For example:
Electronic Gain Control circuits. For related analog circuit examples see:
Op Amp Amplifier with Electronic Gain Control ua-cam.com/video/NoNgQpbj77Y/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
And more amplifier videos are posted in ua-cam.com/play/PLrwXF7N522y5679YAO-lFrNVYqV9XMNTr.html playlist. I hope this is helpful.
👍 A.G.C.=E.G.C.?
Thanks for watching. EGC stands for Electronic Gain Control. AGC stands for Automatic Gain Control. AGC uses EGC circuit in a control loop to adjust the gain or attenuation based on the measured level of signal output. I hope this clarification is helpful. For more EGC examples please see Op Amp Amplifier with Electronic Gain Control ua-cam.com/video/NoNgQpbj77Y/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
Ok sir..