Power Amplifier Design (Class A) with Transformer
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- Опубліковано 30 гру 2023
- How to design a class-A electronic power amplifier circuit with transformer in order to deliver desired electric power to resistor load is discussed in this power amplifier tutorial video. How power amplifier circuit works and how to find the values of bias resistors, the number of turns for transformer and the proper choice of Darlington BJT transistor are discussed. For more electronic and analog circuit examples see • Electrical Engineering... . This power amplifier circuit is designed with one NPN Darlington bipolar junction transistors pair, one ideal transformer and a few bias resistors, bypass capacitors and decoupling capacitor. Both DC bias analysis and AC analysis are discuss for this Class-A Power Amplifier Design to find the DC bias point, maximum power consumption, the maximum collector current and collector-emitter voltage breakdown of the Darlington BJT transistor pair.
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Thanks for watching this power amplifier Design Video. For more examples please see:
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I hope these Circuit design and analysis videos are helpful.🙂
I do enjoy your analysis of these circuits. They encourage me to experiment to see how they perform. This one unfortunately left me hanging a bit since you assumed an ideal transformer throughout the design. Which is of course understandable for the sake of simplicity...and in the interests of time. I would be very keen to see how well such an amplifier might perform in practice from the perspective of THD and Frequency Response. Would you consider taking on the design of a suitable transformer such that a "flat" frequency response might be realised over the audio bandwidth ? This is a non-trival challenge given the need to sustain sufficient flux density within the transformer core without saturation.....
Thanks for watching and sharing your thoughts & suggestions. Glad that that you enjoy my circuit videos and they are useful. As you noted, it is not possible to consider all aspects of the design in the interest of time. To you point, practical consideration and THD of transformer parasitics and non-idealities including imperfect flux and magnetic coupling are important to achieve a decent functional design with acceptable in-band frequency response (say 20 kHz for audio applications). To achieve that the most important step is a properly selecting Audio Transformer or Signal Transformer for the target audio application that can be purchased from either www.digikey.com/en/products/filter/audio-transformers/162 or www.mouser.com/c/passive-components/audio-transformers-signal-transformers/. That proper selection should ensure a practically flat-response transformer in the desired frequency range. If need be, as the next step, an accurate practical spice model of transformer is needed to run realistic simulations in the frequency range of interest. I will post more examples in my analog circuits and signal processing playlist: ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope this is helpful.🙏
Thank you so much. happy new year! Greetings.
You are welcome. Thanks for watching. Happy New Year and greetings to you as well. 🙏🙋♂
Think the darlington vce won't go down too far. The base to collector of 1st would get forward biased.
Thanks for watching and sharing your observations and feedback. You have a good point. The lowest practical pre-saturation Vce for the NPN Darling Transistor is ~ 0.8 volt. We can practically (roughly) neglect that compared with the supply voltage Vcc = 20 volts in this Class-A power amplifier circuit for the approximate DC bias point of Icq = Vcc/(RL*N^2). For more analog circuits and amplifier examples please see ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html . I hope these Analog Circuit Design and Analysis Examples are interesting and helpful. 🙂
BJT junctions typically change -2mV/degC, so over a 50degC change, that's 200mV for a Darlington Vbe. Therefore I'd make Ve atleast 500mV for bias rigidity.
Also batch variations can be +/- 30mV per junction.
Also there'll be asymmetric compression if the source is low impedance and the collector current tries to swing near Icq pk. An emitter degeneration resistor would fix that.
Also it's a good idea to have a base-emitter resistor across the 2nd transistor to swamp leakage currents at higher temperatures and also capacitive AC currents at higher frequencies.
Thank you for watching and sharing your thoughts and practical suggestions. You have good points regarding temperature sensitivity and process variations of PN junction. In this power amplifier circuit while designer has the degree of freedom to choose Re and set it to 2.5 ohm to achieve 0.5v at the emitter of Darlington transistor (instead of the suggested 0.5 ohm value in this power amplifier video) the only drawback is the 5x increase in wasted power in the Re resistor. The initial choice of 0.5 ohm only burns a negligible 20 mW while the increased 2.5 ohm burns 100 mW which should still be relatively ok compared with 2 Watts power delivery target to the resistor load RL. While the access port to the internal node of Darlington transistor might not be available (to insert a compensating resistor), I agree that a small emitter degeneration resistor is a good practice. In this design given the reflected impedance of 100 ohm, a degeneration resistor of few Ohm should practically be OK. I will post more examples in the analog circuits and signal processing playlist: ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Thanks again for watching and sharing your thoughts.🙏
@@STEMprof I assumed Re would be made larger to maintain the same Icq.
It would be interesting to measure the DC saturation current of some small audio transformers.
Thanks again for watching and your comment.
Happy new year! How would you determine the value of C in parallel with Re?
Thanks for these videos.
You are welcome. Thanks Rafael for watching and comment. Happy New Year to you as well. We need to set the value of bypass Capacitor C such that the 3dB cut-off radian frequency (1/RC) is at least 5 times less than the lowest AC frequency of operation for this power amplifier. For more circuit examples please see ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html . I hope the Analog Circuit Design and Analysis Examples are interesting and helpful. 🙂
Your videos have a lot of good information. These videos help me a lot, can you do a circuit on high voltage output (400V p-p) and 500Khz push pull amplifier-- DC to AC. Input from 40 to 50V DC and output pure sine wave 400V p-p, 500KHz.
Thanks for watching, your interest in this engineering channel and your comment. Glad that you like these circuit design/analysis videos and they are helpful. Please see the following high voltage Push-Pull Power Amplifier Design ua-cam.com/video/C9Hse91r5sA/v-deo.html with Op Amp and Darlington Transistors. For more related circuit examples please see: Push-Pull Power Amplifier Design ua-cam.com/video/866MYibo8yE/v-deo.html
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I hope the Analog Circuit Design and Analysis Examples are interesting and helpful. 🙂
Good video. So you got the transformer RL' of 100 and turns ratio of 4:1. So what is the transformer spec? Commercially available is 800:200 ohm. These didn't seem correct to me even though 4:1
Thank you, Glad that you liked this video. Please see the minute 0:11 that states this is an ideal (de-Embedded) transformer in this analysis to just highlight the technique of reflected impedance in transformer circuit analysis. To consider commercially available transformers, we need to separate their impedance and model them as additional passive components at input and output of the transformer and then re-run circuit analysis or simulation. Here are few more Transformer Circuit Videos: ua-cam.com/video/k31t6mWa4Xs/v-deo.html Transformer Circuit Reflected Load Explained: Power dissipation & sinusoidal steady state analysis
also ua-cam.com/video/iCuDWgAde58/v-deo.html video which is an OpAmp + Transformer Gyrator circuit to convert capacitance to inductance.
This topology is of interest to me. Can this design be scalable to 50w with temperature stability? I am curious to see, as an audio amplifier a class A, 50w BJT SE with transformer coupling is similar to the tube version. McIntosh had them in transformer-coupled push-pull class AB but not class A SE.
Thanks for watching and your interest in this Class A power amplifier. Glad that this circuit is useful. For more Amplifier circuit examples please see ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Regarding your question, depending on your target audio bandwidth of interest, it's important to select a high power Darlington BJT pair and also a very low THD Audio Transformer (not generic transformer). I also recommend secent supply bypass capacitors, a slightly higher RE resistor and also considering a reasonably sized resistor in series with the base of Darlington Transistor. Rest of it is just carefully experimenting and iterating. I will post Class AB power amplifier soon in my Analog Circuit playlist ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html . I hope this is helpful. 🙋♂️
If the supply voltage is 20V, how could you expect a 40Vpp as the output at the collector?
Happy new Year from Munich. When the primary gets decreased current (full neg. Current Swing) it acts as generator, negative on the upper side and so positive at the collector side, so collector sees max. 20V induction voltage plus 20V from power supply = 40V (voltage drop across Re was neglected).
@@imunit6666 Happy New Year to you as well. Hope you had a great celebration in Munich Thanks for watching and for your comment. For more analog circuits and amplifier examples please see ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html . I hope these Analog Circuit Design and Analysis Examples are interesting and helpful.🙋
Thanks for watching and your interest in this power amplifier design example. As well-said by @imunit6666 , think of the total voltage at the collector as an AC sinusoidal signal with DC value of 20 volt and with peak to peak oscillation of 40 volts (20 volts above its DC and 20 volts below its DC). Then the maximum collector voltage will be 40 volts (approximately) and therefore the purchased NPN Darlington BJT transistor should have a Collector-Emitter breakdown voltage of higher than 40 volts in order to properly function in this power amplifier circuit. For more analog circuits and amplifier examples please see ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html . I hope this explanation and videos are interesting and helpful.🙏
Hi why is a power amplifier only delivering 2watts . I think I’m unclear on the definition on ‘power amplifier’ thank you for all your videos
@@tristanboyle4450 You are welcome. Thanks for your interest and good question. By definition, a Power Amplifier is a circuit that considerably amplifies the power of input signal to a desired power level at its output high enough to drive a specific load (headphone, speaker, transmitters, etc.). More Amplifier examples are posted in my video Playlist (including 200+ circuit videos). I hope this is helpful.
@@STEMprof so would I be correct in saying that when you have an audio signal at line level it’s only voltage that varies so no powers transmitted so in this case to go from nothing to 2 W is a massive power gain. Is that the general concept?
@@tristanboyle4450 That's about right Tristan 👌. Any considerable power gain especially if driving low impedance output loads would qualify the circuit as a power amplifier.
@@STEMprof you can see the gaps in my understanding thanks so much for your time
@@tristanboyle4450 My pleasure. Glad that this video and discussions are useful.
Nobody today is using transformer for power amp...
Thanks for watching and sharing your thoughts. While use cases of transformers are limited, there are still audio and automotive power amplifier applications using transformers. That is why Mouser and Digikey still distribute and sell transformers www.digikey.com/en/products/filter/audio-transformers/162 and www.mouser.com/c/passive-components/audio-transformers-signal-transformers/. For more circuit and Op Amp examples please see Op Amp Amplifier with Electronic Gain Control ua-cam.com/video/NoNgQpbj77Y/v-deo.html
Push-Pull Power Amplifier with Darlington Transistors ua-cam.com/video/866MYibo8yE/v-deo.html
Instrumentation Amplifier with Electronic Gain Control ua-cam.com/video/C4tghZ-q6Zs/v-deo.html
5-volt Charger Voltage Regulator with Temperature Compensation ua-cam.com/video/NdfyHoxjKrY/v-deo.html
For more analog circuits and signal processing examples see: ua-cam.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Thanks for watching.
Transformers that are listed are not used in power amplifier except tube amps!@@STEMprof