#2612 NE5204 SA5204 High Frequency Amplifier (part 2 of 2)

追加: 2026-06-03

[00:00:06]All right. I was editing the video I did yesterday on a NE uh 5204 and I kept saying you know you should read the data sheet. You should read the data sheet. You should learn from it.

[00:00:17]You should use it as a textbook basically. Um and I thought well why am I saying it? Well let's let's do it.

[00:00:24]Let's give an example of how you can learn from a data sheet. Okay. So, um, we're going to go through this theory of operations. So, it's going to tell you how this circuit works. At first glance, this circuit seems to be pretty crazy, but it tells you, you know, what the designer was trying to do and why he did it, what these different components do.

[00:00:46]And so, the first thing we need to do is to kind of back up and take a look at what is the basic basic basic idea of this amplifier. And it is this. Okay. So this is the core of the amplifier. We have two transistors input and output.

[00:01:01]So this one uh amplifies it to a certain extent and then this one takes that and amplifies it a second extent. Right? So there's a beta in this transistor and there's a beta in that transistor and they multiply together and yeah you get you get gain, right? It's like a two-stage gain. Um but this as it stands would sort of have infinite gain. Uh you can imagine if this transistor is off then this one is on all the way. It's going to it's going to turn on, right?

[00:01:31]And if this is on all the way, then this one is off all the way. And so it's going to be railto rail bang bang. Uh it would be just kind of out of control.

[00:01:41]And so all of this other stuff makes it controlled. Okay? It it puts dampeners on everything, right? It puts the limits on everything. So uh but this is what you need to first understand. If you don't understand this then maybe look at some other documents and stuff on how transistors work. I've done some videos but um uh you know you wiggle here it's going to the these things work in current. So if you have current coming through here it gets multiplied. So a little bit of current here ends up in a lot of current here and you get wiggle here and then that affects the current that comes this way.

[00:02:18]Sometimes the current goes this way.

[00:02:20]Sometimes the current goes this way and you get current splitting. Um so anyway wiggle it here gives you a wiggle in in in this current in this uh base here and then that gets amplified by the beta transistor and you get bigger wiggle here. So wiggle wiggle.

[00:02:39]Okay. So the first thing we're going to take a look at is um the what is the gain of the amplifier.

[00:02:48]Okay. And uh so a lot of times you have to go ahead in uh learning and get a concept and then come back and figure out how that worked. Then kind of go ahead and then come back. So sometimes learning isn't linear. You sort of have to flop back and forth and keep things in your head. All right. So the first thing they talk about is what is the what is the what is the gain? All right.

[00:03:15]And um I put a link link down below with this uh data sheet so you can follow along have it not having to read on the screen. Um so V out over VN they claim is RF1 plus RE1 divided by RE1. Okay. So what does that mean? All right it's these it's this pair of of resistors here. This is RF1 and this is RE1. So it's a resistive divider. Okay. And so you have this plus this um divided by this. Okay. So you have 140 + 12 / 12. All right. And that is the gain. All right. So let's do that.

[00:04:00]Okay. So 140 + let me use a pen. It shows better on screen.

[00:04:08]140 + 12 / 12. I say that is that is the gain and let's find a calculator and we will do 140 + 12 / 12 the gain is 12.66.

[00:04:28]Okay.

[00:04:30]And uh jumping ahead a little uh that would be 20 log of 12.66 66.

[00:04:41]Um the gain would be let's see take the log of that uh log base 10 uh times 20 and so we would have 22 dB of gain.

[00:04:57]Okay. So we have 22 dB of gain. This is how you do decibels. You take the logarithm.

[00:05:03]All right. So uh so we've we've we've seen how that might work. And you say, well, okay, okay, so this this does the gain. Well, how does that work? Okay, so so that one concept is the gain, and you know that this sets the gain. And I said before the gain is kind of infinite here. So we've kind of cranked it down to 22. Used to be infinite gain, but now we've cranked it down to 22 dB. Well, how does that work? Well, we take the output and we feed it back to the input.

[00:05:32]So this resistor takes some of the output and feeds it back in. All right.

[00:05:37]So, uh the we had remember we had current wiggling here in the first transistor and this will come back around and it will affect that. It will subtract a little bit from that. Right.

[00:05:49]All right. So, how do they do that?

[00:05:51]Well, they do that with this transistor and this transistor. All right. So, let's go ahead and read what it's what it's doing here. It talks about noise figure. I'm going to skip that from now.

[00:06:02]I think that's kind of jumping the the the cart. Okay. Um, so it says DC voltage level can be determined by All right. So it talks about um an emitter follower Q3. All right, I'm I'm up I'm reading in this paragraph here. It talks about Q3 and Q4. Okay, that's these two guys. So uh this uh transistor is an emitter follower. It's taking the output. This is the output comes into the base here into into the emitter. So that's emitter follower. and then it goes through another diode. So it's emitter follower through a diode. Um and the reason they put in the extra diode is for a DC condition. Now whenever you analyze amplifiers, you have to analyze them with AC gain and then with DC biasing. Okay? You need to do a DC analysis and you need to do an AC analysis. Okay? and they did the DC analysis and they said um okay the DC voltage out here when it's just sitting there and you don't have any inputs and outputs where do you want things to be okay and so they said well we want the input to be a certain level and want the output to be a certain level and what they want the output to be is they want be the output at half of VCC so if VCC is say 6 volts they want the output to be at 3 volts and then they can go up and and they can go down so they can wiggle wiggle And they can get maximum wiggle wiggle by setting the DC condition here at 3 volts. And the way that they do that is they take the 3 volts here. They subtract it with a diode drop and they subtract it with another diode drop. Okay? And uh then they say okay uh let's figure out what the DC condition of the input is. Right?

[00:07:52]The DC condition of the input is this equation. They said the DC condition is VBE. Um that's this voltage drop here.

[00:08:01]That's a VBE. So whatever your input, you're going to drop it uh a diode drop here. And then it says it's uh IC1 plus IC3 over RE1. So what does that mean?

[00:08:12]Well, it means if you take a look at this resistor here, we have current this way. We also have current this way. So IC1 is the current going through Q1. IC3 is the current going through Q3. So you have some current going through here and you have some current going through here. They they they share the R1 shares current. Okay? And so you have this current plus this current across that resistor. That's a voltage, right?

[00:08:37]Current across the resistor is voltage.

[00:08:40]And they do they give you they give you the values here. Okay? And you end up with one volt. So you end up with one volt of biasing on the input. So the input is one volt. And so your wiggle wiggle here is moving one volt up and down just a tiny little bit. Right? It's current input. All right? So one volt here. Okay? And then you have 7 volts here. And then you subtract that 7 volts. You subtract that 7 volts and you have a resistor divider. And you do all of that. You end up with 3 volts on the output. Okay? So they give you all that equations here. They give you all the conditions. They say uh you have a 12volt res 12 ohm resistor. VBE is8 volts. Okay, I said 0.7 but they had8 volts. Uh one current through the first transistor was 5 milliamps and then through the third the Q3 was 7 milliamps. So you have a total total of 12 milliamps and you go through the 12 ohms you find out you're at 1 volt. Now the level shifting is done by using the Q3 and the diode Q4. That's this one and this one. Okay. Provides uh feedback, okay, through RF1. That's that guy there. Okay. And they give you the calculation here. They say the output, okay, is going to be VCC minus whatever current you have in going through R2.

[00:10:01]Okay.

[00:10:03]And R2 is this one over this one's over here. It's also 12 ohms. Uh oh, no, R2.

[00:10:10]I'm sorry. they they do the the uh they're doing the calculation on the high side, not the bottom side. So they say, okay, whatever voltage you have here is this minus whatever voltage you have across this resistor. That's R2.

[00:10:22]That's the 225 ohms. And so they they say you have 6 volts on the on the VCC.

[00:10:28]You have 225 ohms. And then once again, you had um let's see here, you had IC2 and IC6.

[00:10:39]Okay. So, you have you have the current going through the oops, sorry. You have the current going through this uh uh transistor and you have the current going through this transistor. Now, we didn't talk about this. Remember, I just drew it as a simple one transistor output. But if you needed more gain, you could you could just change this this uh transistor to a Darlington. And that's what they did. They put a Darlington here. So you can just imagine that this uh right here was just was just one one IC was one can was one to t to 92 package. It was a Darlington transistor and that's what you put in here. This is just the Darlington, but you have current this way and current this way.

[00:11:22]So they're adding them together. Anyway, they do the calculation and they find out you have about 3.3 volts of biasing here. Okay, so what did we learn? Okay, we learned that, okay, it's a Darlington that gives you more gain on the output, but it's the same. Uh, you calculated you have one volt on the input and you had 3.3 volts on the output. And then you calculated that whatever wiggle wiggle wiggle here is uh buffered with this transistor. Okay, it talks about that. It says you don't want the wiggle wiggle here being affected by whatever wiggle wiggle here. So, you buffer it.

[00:11:59]So this gives you a high impedance uh uh buffering to do the feedback loop. Okay?

[00:12:06]And so you run that into this 140 ohm resistor. And that sets up your nominal gain. That resistor divided by that resistor. Okay? So now we know why there's two transistors here. Now we know why there's a transistor here. Now we know why this diode is here. And now we know what all of this does. Okay?

[00:12:26]Now, we just need to figure out, well, what's going on down here? Why why do we have an extra an extra diode down here?

[00:12:33]What is what is that doing? Okay. Uh, let's see here. And it talked about that. It talked about Q5. Here we go.

[00:12:40]Uh, it says we can see that the output voltage is 3.3 volts relative to the positive input blah blah blah blah. Q5, this diode down here, is included for bias purposes to allow direct coupling of RF2 to the base of Q1.

[00:12:57]Oh man. Okay. Uh RF coupling to the base. Okay.

[00:13:08]If we look at RF2, you go, wait a minute. What's that doing down there? We know we already have feedback. Why do we need why do we need this resistor? It feeds back to the input. It looks like this. There's like there's two feedbacks. That's weird.

[00:13:21]So it talks about dual feedback loops to stabilize the DC operating conditions.

[00:13:27]So one wasn't quite enough. They needed one to balance a little bit more and they needed this diode drop to to set the DC conditions correctly for this feedback loop. Okay. So there's two feedback loops.

[00:13:41]All right. And then it talks about the output stage is Darlington pair which increases the bias voltage on the input stage to a more desirable value and also increases the feedback loop gain. Yeah.

[00:13:50]Okay, the resistor R0 optimizes the output visor. What's the R0? They put a little 10 ohm resistor here just to kind of buffer the output, make make it closer to 50 ohms. That's nice. And then L1 and L2. Remember I said there's you get need to account for kind of inductances on the leads and stuff like that. These are three nanohenies. Three nanohenies. It talks about why they put why they include those in the calculations. They say those are the bond wires and lead inductances on on the dip package. Right? So the bond wire I have to include the bond wire also.

[00:14:24]The little bond wire and the leads are roughly three nanhenre's and those inductors improve the high frequency impedance matches at the input and output partially resonating with a 0.5ar of pad and package capacitance. So they're having to actually worry about pads on the PC boards and lead lead inductances and all that kind of stuff.

[00:14:46]When you run at higher and higher frequencies, you need to make sure you're accounting for everything. So they've they've included the uh the lead inductance on their calculations.

[00:14:57]All right. So I hope that was somewhat instructive. Uh we sort of figured out, you know, why why it's so complicated when actually when actually it's just this, right? It is just that. But again this would have like infinite infinite gain and instability and this limits the gain limits the limits the DC uh to wiggle around 3 volts. This would go whack whack whack whack you know plus and minus to ground to plus ground to plus. This limits it to 3 volts and wiggle wiggle around 3 volts by having these two feedback paths. And they needed two feedback paths to manage this thing. Sometimes one is not enough and in this particular case they have basically the AC gain is set by these by these two resistors and then some of the DC biases is helped by this other resistor at the bottom.

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