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I'm trying to amplify a 20mV ptp signal to something around 1V ptp. I intent on only doing this in one stage and require the amplifier to be very low power consumption (3.3V supply max 1mA for entire amplifier). I'm trying to choose a bias point to operate the device at that will give a low enough power consumption and still give me the optimal gain I'm looking for. Drawing a load line on the BC846 NPN BJT I get a Q point that is very hard to achieve: enter image description here

I was wondering what characteristics I am looking for in a BJT that would give a lower current bias point.

My current amp design: enter image description here


The bandwidth is only 100kHz.

Source impedance is ~5k purely real and load impedance is 100k real.

Dave Tweed
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  • What bandwidth is needed? This will help you choose the best approach . – Autistic Oct 23 '19 at 10:29
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    Although they teach this simple common emitter configuration in schools all over the globe, it will not ever yield the performance you are likely to require. Not having an emitter resistor means very poor dc stability with temperature and this also means poor gain accuracy and also significant distortion. My personal rule with BJT stages is never try to exceed a voltage gain of ten and, you are asking for fifty. Of course I break my own rules now and then but, I know what I’m doing. – Andy aka Oct 23 '19 at 10:35
  • Everyone above my comment has presented points that you should attempt to address. One huge glaring problem is that you face very high distortion and probably should consider at least a second BJT as some form of global NFB. Or an opamp. You don't have much overhead voltage to work with and the high voltage gain makes things very problematic. You could consider bootstrapping to help in some way. But even there, your problem is bounded on all sides by difficult compromises. – jonk Oct 23 '19 at 13:06
  • Thanks everyone for the replies and advice. To answer a few questions: The bandwidth is only 100kHz. As for using two bjts to achieve better stability per stage I think this is the approach I'll investigate. Source impedance is ~5k purely real and load impedance is 100k real. Finally, @jonk I'm not sure I understand what a global NFB is. – Mackenzie Goodwin Oct 23 '19 at 13:29
  • @Mack Global NFB is just negative feedback that spans more than one stage. For grounded emitter designs it is vital. There are many different and interesting 2-bjt topologies for something like this. So don't be surprised by very different looking arrangements and don't stop at the first one you see, either. 100kHz seems to add even more trouble. But I'd have sit down to find out for sure. I haven't needed Av=50@100 kHz on 3.3 V rail with one BJT and no source impedance spec or load spec. I fear you will need to write out lots of detail so others can make back of the envelope estimates. – jonk Oct 23 '19 at 13:40
  • Also, since you're AC coupling the thing, say what your low-frequency limit is -- i.e., tell us the upper **and lower** frequency bounds of your amplifier. – TimWescott Oct 23 '19 at 14:08
  • You have choosen Rc=2kOhm, so in a homework context your next task is to bias Q1 for an Ic=625 uA yielding a gm of \$\approx 25\$ mA/V. – sstobbe Oct 23 '19 at 15:50
  • with 4mvPP input, on grounded emitter, you'll have 10% distortion (if I recall rightly). With 20 mVpp, you'll have over 2:1 variation in gm (18mV is exactly 2:1), causing obvious POS versus NEG distortion of the SIN peaks. For grounded emitter and a passive Rcollector, the maximum gain you will achieve is VDD/0.026; in this case, 3.3/0.026 == 130 or so. A gain of 50 should be easy. I'd include a discrete Re, 51 ohms? to reduce distortion. – analogsystemsrf Oct 23 '19 at 17:52
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    @analogsystemsrf Avoiding *deep* saturation of the BJT would probably mean (3.3-.6)/.026 or Avmax of about 100, not 130. We still have no idea what this thing is supposed to drive, nor what's driving it, nor what distortion can be tolerated. And there will be attenuation on both ends, whatever it is. I think the gain of 50 won't be a cakewalk. I suspect there will be some carefully crafted trade-offs to get to a realistic end point. – jonk Oct 24 '19 at 00:56
  • @analogsystemsrf Something else just crossed my mind to make this more difficult. That calculation of Avmax is based upon maximizing the gain on the positive-going half-cycle. But biasing a BJT towards the very edge of its saturation region doesn't allow room for the negative-going half-cycle. So it's now looking still tougher to me. I should have seen this before. But I'm just a hobbyist. Not a professional. So I miss the obvious, sometimes, and it takes time to drip through. Oh, well. – jonk Oct 24 '19 at 06:59
  • @Mack What's the frequency range for that bandwidth? Not because I think it matters for a single BJT amplifier like this, since the other requirements, so far, now appear to be unachievable. Also, how much distortion can you tolerate? (You could answer that, at least partly, by simply telling me the full, allowable range over which the voltage gain can move throughout one cycle at the input, I suppose.) – jonk Oct 25 '19 at 16:02

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I think I can summarize all the comments from above into one answer: Use an Opamp for this task and forget about the single stage BJT. The Opamp's properties reliefs you from a lot of trouble you would have to deal with when using a single BJT.

Stefan Wyss
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Original Post

This is the circuit I think you are inquiring about:

schematic

simulate this circuit – Schematic created using CircuitLab

As you can see, there are some specifications. But to be honest, not enough. No idea of \$R_\text{L}\$ or \$R_\text{S}\$ (which cannot be ignored), for example. Suppose \$R_\text{L}=3.3\:\text{k}\Omega\$? Then already you have \$\pm 1\:\text{mA pp}\$ as a compliance requirement for the output. And that's without the amplifier itself requiring anything at all to support itself. It brings up another question, too. Do you mean \$1\:\text{mA}\$ as the average current from your \$V_\text{CC}\$ supply rail? Or do you mean \$1\:\text{mA}\$ as the worst case peak value? Not specified. Yet it probably is important to know. I have no idea what the frequency is supposed to be (your schematic's AC supply says \$0\:\text{Hz}\$; but of course I'm sure that's not correct.) And no idea what the bandwidth should be, either.

Let me cut to the meat of things, though. Let's assume that you are willing to bias \$Q_1\$ in such a way that it sits just on the cusp of saturation -- say, where \$V_{\text{CE}_\text{MIN}}\approx 500-600\:\text{mV}\$. And let's also say that you are willing to have the collector voltage swing all the way to the \$V_\text{CC}\$ rail, as well. That's the absolute maximum possible collector swing. Then it follows that \$A_{v_\text{MAX}}=\frac{V_\text{CC}-V_{\text{CE}_\text{MIN}}}{V_T}\$ (assuming the usual emission coefficient of \$\eta=1\$.) At a maximum operating temperature of, say, \$50^\circ\text{ C}\$, \$V_T\approx 28\:\text{mV}\$. So this suggests \$A_{v_\text{MAX}}\approx 100\$.

That may sound good to you. But there's a problem. This only works with the BJT biased right at the very cusp of saturation. Which means that only one half-cycle of the input AC signal can be passed along. The other half-cycle is almost entirely clipped away, as it drives the BJT deeply towards saturation. And it ignores the huge distortion caused by widely varying gain for the half-cycle over which it does work.

Oh. Did I forget to mention that biasing the BJT to the very cusp of saturation, in the grounded emitter configuration you show, is quite temperature-dependent? Well, let me mention it now. That's another problem. Not much thermal stability. Even assuming you could accept its other difficulties.

I'm almost certain that's unacceptable. Which means the above calculation of \$A_{v_\text{MAX}}\$ is grossly overly optimistic. So now it seems that there may be some difficulties achieving your voltage gain goals, even temporarily ignoring the rest of the information you've failed to provide.

I have tried here to illustrate what's missing in the information you've provided and pointed you towards the singularly glaring difficulty in achieving a high voltage gain with a relatively low \$V_\text{CC}\$. I've also pointed out that there will be a great deal of distortion involved when pushing this to its limits. And I've added that all of those factors will also be temperature-dependent, which means the circuit will do different things in different seasons or areas of the world.

So you need to clarify the temperature range over which this circuit must operate, how much ripple or allowable variances there might be in \$V_\text{CC}\$, whether or not the \$1\:\text{mA}\$ current limitation is average or peak, the bandwidth required over which it must operate within these specs, the load and source impedances, etc. There's a lot missing in this. I don't see how anyone can help you without writing an entire textbook along with a set of course study materials and lesson plans.

Additions Due to Updated Question

You've added that the bandwidth is \$100\:\text{kHz}\$, the source impedance is \$5\:\text{k}\Omega\$ and the load impedance is \$100\:\text{k}\Omega\$. This still leaves out the start/end range for the frequencies of interest and it doesn't discuss what kind of distortion you can accept.

But I've something I'd like to add, as well.

For any kind of thermal stability in this general topology (by this, I'm allowing for similar circuits that include some local NFB via emitter degeneration resistors), this relationship must be true:

$$V_T\ll \frac{V_\text{CC}-V_{\text{C}_\text{Q}}}{A_v}$$

Let's assume a worst case \$V_T\approx 28\:\text{mV}\$ and your existing \$V_\text{CC}=3.3\:\text{V}\$. Let's also assume two values for \$V_{\text{C}_\text{Q}}\$: \$V_{\text{C}_\text{Q}}=2.1\:\text{V}\$ (which in a "normal" design, it might be) and \$V_{\text{C}_\text{Q}}=0.0\:\text{V}\$ (which it cannot be in any realistic/useful design.) We get the following two relations:

$$\begin{align*} 28\:\text{mV}\ll 24\:\text{mV}\tag{false}\\ 28\:\text{mV}\ll 66\:\text{mV}\tag{likely false} \end{align*}$$

I don't consider either of these to be true. The first case is obviously false on its face. The second case is false once you add in the fact that it derives from an impossible condition -- that we can set \$V_{\text{C}_\text{Q}}=0.0\:\text{V}\$ and have a useful (or buildable) amplifier. So either the condition is false by itself or else it is falsified by a false prior assumption. Either way, the whole idea loses out.

(By the way, you can derive the above relationship on your own by merely laying out the equations for \$I_\text{Q}\$, \$r_e\$, and what you'd need to do in creating some emitter degeneration to compensate for temperature.)

This leaves us with terrible compromises to consider. We can adjust \$V_{\text{C}_\text{Q}}\$ upward, thereby moving the above computed relation towards the very limits of any validity, while at the same time avoiding an out-and-out violation of it.

For example, we might say \$V_{\text{C}_\text{Q}}=1.0\:\text{V}\$ and find that \$28\:\text{mV}\ll 46\:\text{mV}\$ and figure that might be as good as we can get in this circuit. But this means that \$V_{\text{C}_\text{MIN}}=500\:\text{mV}\$ and that is already pressing the BJT into saturation and doesn't really allow us any room for emitter degeneration. But let's say we can accept \$V_{\text{CE}_\text{MIN}}=400\:\text{mV}\$ (arguably not quite yet too deeply saturated), thus allowing ourselves about \$100\:\text{mV}\$ for \$V_{\text{E}_\text{Q}}\$. Then, we find that the base biasing pair of resistors as well as \$r_\pi\$ will probably attenuate your input signal by at least 50%, destroying the goal as we try so hard to save it.

This means to me that you'd have to go to a bootstrapped design (in order to reduce the input attenuation) in order to have any chance. But you'd also have to accept other compromises (gain variation and thereby perhaps unacceptable distortion.)

If you specify more about what kind of distortion you can accept and address yourself to the above comments in my answer a little more fully, I might attempt a bootstrapped design. But only if you tell me that you can fully accept the host of compromises that such a design implies and that a bootstrapped approach doesn't otherwise violate your requirements. (It adds one resistor and one capacitor.)

jonk
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As suggested by many people (thank you all for the suggestions) I have designed a multistage amplifier for the solution using a source follower to buffer the input (due to large source impedance) and then a two stage class a then class b amplifier.