EMRFD Message Archive 15360

Message Date From Subject
15360 2019-08-10 11:40:48 David Feldman Audio stage in "A Minimalist Direct Conversion Receiver" - design/mo
In my copy of EMRFD (first edition, (C) 2003), I am reading about "A Minimalist Direct Conversion Receiver" (Page 8.4-8.5, Figure 8.7) which incorporates an audio (pre-)amplifier stage consisting of a single NPN transistor in common emitter, with two bias resistors (one base-to-collector, in this example 150K, and one collector-to-supply, in this example 4.7K) and inter-stage blocking capacitors.

I would like to learn how this amplifier stage gain is set (presumably a function of only the transistor and the two resistors), or get an intuitive idea of the idea behind the function of the collector-to-base resistor.

This same type of stage is used in the Ubitx project's receive audio stages, and my derivative of Ubitx version 2 needs some additional receive gain (perhaps 6-10 dB), preferably working within the confines of this single stage. The designer of ubitx pointed me to DC-40, which took me to EMRFD and this article which embodies the stage in a design that is simpler than the Ubitx, but I've not found anything in EMRFD book that explains the design behind this circuit stage.

Thanks for any advice/pointers.

15361 2019-08-10 11:59:16 Ashhar Farhan Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
This audio preamp is running flat out. The biasing arrangements provides very little feedback to have any effect on the impedance and gain. Instead, it biases the transistor collector to half way between the ground and the Vcc. 
You can simulate this with the fba.exe program on the CD that accompanies the book.
73, f

15362 2019-08-10 12:17:18 wb0gaz Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
Thanks, and some progress ---

I found some useful information in EMRFD (2003) Page 2.4 Eq. 2.9 and Figure 2.12, which give me an intuitive idea of what the amplifier is doing (R1 is doing duty as both bias and negative feedback element). Use of Eq 2.9 and some measurements tell me Beta of the MMBT3904 is 180 in my implementation of this stage (just comparing quiescent base and collector currents).

Unfortunately, the text discussion ends with Eq 2.9, so I do not see how to proceed to calculate the stage gain (which perhaps is influenced by impedance match on both input and output and hence cannot be calculated independently.)

I had initially hoped (but so far not succeeded) that using a higher gain device would yield (at least somewhat) higher stage gain on the theory that Beta would dominate the stage behavior; this didn't pan out experimentally. The MMBT5089 I tried (vs. MMBT3904) was part of a kit assortment I bought long ago (so it was on hand), and in a quick search, seems like one of the higher gain options in SOT23 package. Substituting MMBT5089 did change operating point, and I calculated Beta of the MMBT5089 is 560.  Unfortunately, increase of beta by 560/180 ~= 3 did not result in a corresponding increase in measured noise voltage at the loudspeaker (only about 10% increase in Pk-Pk noise voltage.)

There is no sign of clipping in the loudspeaker output voltage for either device in Q70 stage I tried.

I am trying (as best I can at this point) to find a solution that fits in the existing (PCB) footprint, hence my persistence on this topic.

Thanks again,


15363 2019-08-10 12:28:07 Zach Metzinger Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
15364 2019-08-10 13:15:56 Zach Metzinger Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
15365 2019-08-10 14:34:49 Ashhar Farhan Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
See this http://w7zoi.net/transistor_models_and_the_fba.pdf
It was written by Wes, of course

15366 2019-08-10 14:42:14 wb0gaz Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
Zach, Ashhar

This is all making much more sense now.

I actually put a MMBTA13LT1G into the circuit earlier this afternoon; found that the output voltage at the speaker slightly declined from my original test.

At this point, I realize that each time I've installed a higher-gain device, I've failed to recalculate the bias/feedback resistor's value, and in each case Vce has been very low (the Darlington appears to be in or very close to saturation). It appears that with too-low resistance in the feedback path, the transistor gravitates towards a near-saturated state

Next step is to focus on getting quiescent Vc somewhere in a workable mid-point between 0 and VCC.

Thanks again,

15367 2019-08-10 14:52:01 Ashhar Farhan Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
we often look for better device performance when, infact, we should be looking for a better circuit design performance. it would be better if we could a second stage and have a stable amplifier. 
If you consider the audo preamp with the common base configuration, it is really a brillinat design. The half milliamp in the bias current automatically sets the input impedance at audio at a strong 50 ohms, a good match for the preceeding diode demodulator. That stage's gain is set by the ratio of RL and Re, removing device dependence. the second transistor is a common emitter stage, directly coupled. the second stage's gain is again set by the ratio of the emitter degeneration to the collector load. By dividing the gain into two controlled stages, the gain is stable, the match is good and the isolation is quite strong.

73, f

15368 2019-08-10 14:56:47 wb0gaz Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
Agree of course - I am trying to force the existing design to behave somewhat better, as I am working with an existing PC board and trying work out all of the various issues (these were not problems in original ubitx, but rather all artifacts of my learning curve) before doing a new version of the board. Clearly before the next version I will prototype a new first-stage amplifier (your references have pointed me in good directions).


15369 2019-08-10 16:17:42 w7zoi Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
Hi David, and gang,

With few exceptions, the gain of a common emitter, bipolar, small signal amplifier has little  to do with transistor type, or transistor beta.    Rather, it is determined mostly by the bias current that flows in the emitter and by the collector load.   DC beta will have a small impact on the bias current.

Back in the days of vacuum tube amplifiers, we thought of our amplifiers as being voltage driven devices.   The voltage on the grid of a grounded cathode amp controlled the signal current in the cathode.   Most of that current also flowed in the plate, making it easy to calculate the voltage gain.    The input impedance at the grid was quite high, allowing us to generally ignore it.

Then bipolar transistors came along.   They had current flowing in the base.   Beta was thought to be a big thing and we would treat these amplifiers as current controlled designs. But this often complicates things more than is necessary.    In reality, we can treat our amplifiers as being either current controlled or voltage controlled.     I've moved away from the current control toward a more traditional voltage control in my thinking.   The equations are simpler.   Moreover, I've observed that this is the way it's done by most of the experienced circuit designers I encountered back when I worked for a living.

So to analyze the audio amplifier, you calculate the standing DC current.   This then gives you the intrinsic emitter resistance rd=26/I where I is the emitter current in mA.    The gain is then the ratio of the collector load to rd.     There was  a 4.7K collector resistor.   The actual load is this in parallel with the input impedance of the next stage.

Beta does have some impact, but only indirectly.   Beta determines input impedance.   If you are driving the stage from a high impedance source and the input impedance is low because of a low beta or the biasing, etc, the voltage on the base will be below the level that would be there if you pulled the transistor out of the circuit.   But the relationship between base signal voltage and the emitter signal current is still controlled by rd, which is set by the bias.

There are other details in the amplifier.   The 150K resistor is mainly a bias element, but it also provides a bit of negative feedback.    If a capacitor parallels the 150K, that will also provide negative feedback, now frequency dependent.   The value of blocking capacitors can impact things a little.

These methods are used to analyze some feedback amplifiers on my web page, http://w7zoi.net/     Once there, go to the "technical notes" and then to the "EMRFD Related  Stuff."    There are three files grouped in a heading "Small Signal Amplifier Design."    The one titled "The Feedback Amplifier with a Simple Model" is the one with the math.   

Anyway, I hope that this helps.

73, Wes

15370 2019-08-11 08:30:33 wb0gaz Re: Audio stage in "A Minimalist Direct Conversion Receiver" - desig
Thanks very much - that is very helpful. I had about the same observation here after trying several bias settings vs. several different device types. Will study the references before heading down this path again,