Amplifier with gain control
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- čas přidán 8. 10. 2020
- See how a JFET can be used to control the gain of a transistor amplifier.
In this circuit a constant current biased common emitter amplifier has its gain controlled by a JFET used as a variable AC resistor. - Věda a technologie
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Thank you for taking the time to explain these circuits! Love to learn something new.
Best explanation of this circuit I have heard! 🙌
You have a very good channel. I've just discovered it. Congratulations. Greetings from Spain.
Thank you! I appreciate your help to grow it 💪
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Great video, thank you
Ótimo vídeo! Me ajudou em um projeto para a minha graduação em engenharia eletrônica.
Thank you!
i like this! :D
thank you
Hello thanks for you vidéo,please wath is the rule both 220k?
I like it! Thanks.
I'm glad!
Intresting, can be used as mixer as well
Nice explaination
Thanks for liking!
Hey, this is a fantastic video... I'm curious, is it possible to do something like this without the negative supply rail (a single supply)? I have been working through a bunch of possible changes in LTSpice, and can't find a way to make it work.
Thank you!! I think it is! But you nees to bias the signal at half supply (vcc/2)
@@AllElectronicsChannel OK, thanks. I think I have something sketched up on ltspice along those lines that might work. Now I have to order some JFETs...
Great video! loved it. Wonderful circuit. Are there consequences to changing the 56ohm to say 1k so I can drive the circuit it more easily from say a typical consumer audio line out impedance?
Welcome! No problems! I used 56 to better match with the signal gen
@@AllElectronicsChannel Thankyou!
Nice little circuit. Since the bias changes, I was wondering if the THD+N also changes with it?
The DC bias actually do not change, because the JFET has it's drain AC coupled. But.. The JFET variable resistance is not much linear, so it will be the main source of distortion!
@@AllElectronicsChannel Thanks. Yes that was basically what I was thinking. For the JFET, I think you mean that the modulated signal is distorted?
The two signals are distorted by two different reasons!
1) the modulated signal is distorted because the JFET channel resistance will be nonlinear because of early voltage (channel modulation) [the AC signal will be present at the JFET as a Vds and it will modulate the channel resistance]
2) the modulating signal also is "distorted" (the modulation index is not linear) because the relation of gate voltage to channel resistance is not linear
The first effect is minimized by a first order compensation by the two resistors you can see in the JFET. I used a voltage divider between drain and gate so half the AC drain voltage is summed to the gate voltage. This cancel (in a first order manner) the squared therm in the equation of channel resistance that is responsible for the channel modulation effect).
Even with all that, this circuit will work very well and is useful for signal modulation and automatic gain controllers!
have more data on peak detector?
thanks
Hi, maybe you know solutions for such question:
Need to design local oscillator with stable output power and simplfy calculating systems i design, for example, i use 100MHz crystal osc. The amplitude of output signal of osc. vary from one crystal to another from -3dbm to +13dbm, so to compensate this was decided to put an rf amplifier working in compression. This was made to stabilise output power. But there is one more parameter called power saturation. So for example in my rf amplifier (adl5545) compression point is nearly 15dbm and gain is nearly 24dbm. When amplitude of osc. is -3dbm the output power of amplifier is 14 dbm (p1db) . But when amplitude of osc. is 7..13dbm the output power of amplifier is 19 dbm - is not a compression point (is power saturation point). But in datasheet there is no information about it.
So how to design stable power output of rf amplifier does not depending on input power?
STABLE output power is only possible with a negative feedback servo-loop around the amplifier. You can have a signal diode measuring the output amplitude, feeding back a correction signal to the amplifier bias. Something like that will work with constant output power, sometimes this is called "levelled amplifier"
Very nice..! Wat must be change to make it work as an dsb-sc modulator? Only the bias or is more needed? Gr Fred.
Thanks! Yep, it probably gonna work !
2n5531...is this marking correct??? And For both transistors? Becorse, the 2n5531 is an metalcan transistor. Gr Fred.
@@fredzwijnenberg5361 hum!! I think it is 2N5551 !
THANK YOU!!!!
👍🤘🤘
I notice that the circuit has an asymmetric power supply, +12V, -5V. Would it be possible to achive the same performance with a symmetrical supply, and what changes would be required?
The asymmetrical supply was a design challenge in one class of a Hotmart course I'm lecturing. For a symmetrical supply you only need to change the bottom current sink BJT bias. If you bias it for constant current and linear operation, it will work !
@@AllElectronicsChannel Many thanks for the reply. :)
Newbie question here: On the gate of j-fet I can use a trimpot or potenciometer ?
Yes, you can !
What happens if you modulate the current sink?
Gain will be changed but also the DC bias point!!
Nice circuit!! just a comment, Gain can't be zero as it is usually defined as Av=vout/vin. if it doesn't amplify, Av = 1
0/vin = 0
@@AllElectronicsChannel ha, true that. But anyways you still want signal in the output.
Hahahaha thank you for watching!
circuit was nice but i hv to wear soda glasses or telescope to see clearly
It would be nice if you could complete the schematic,
so you can actually use it.
now you have nothing to do with it!
🤯