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Impedance matching for the audio stage
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Richard Seguin
Hello,
I'm designing an audio stage after a BFO diode ring mixer and I seem to be having trouble converting a 50-ohm impedance to something the amplifier can handle (10K input). I know how to convert impedances with RF signals using toroids, and I assume that the exact same would apply to audio. Does anyone know of a toroid or ferrite core that is decent at audio frequencies? Richard (VE1XOR) |
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the output should be audio.? a small xfrmr of what ever impedance you need,? should do the trick.? ebay has a lot
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dave k7da On 5/20/2019 12:41 PM, Richard Seguin wrote:
Hello, |
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It's fairly important that the DBM see 50 Ohms. The master for matching these product detectors to the audio chain is Rick Campbell KK7B who designed the R1, R2, miniR2 and microR2 receivers among others. Here's a link to the R1 <>. Just seach on kk7b for the others.
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Hope this helps. 73, Pat? ? VE3EUR On Monday, May 20, 2019, 3:41:52 p.m. EDT, Richard Seguin <richard.seguin@...> wrote:
Hello, I'm designing an audio stage after a BFO diode ring mixer and I seem to be having trouble converting a 50-ohm impedance to something the amplifier can handle (10K input).? I know how to convert impedances with RF signals using toroids, and I assume that the exact same would apply to audio.? Does anyone know of a toroid or ferrite core that is decent at audio frequencies? Richard (VE1XOR) |
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ajparent1/kb1gmx
THe most common is one of two... Transformer from 50 ohms to say 500-100 ohms to
match the base of a transistor. The problem with that is bandwidth as its a poor match across the DC to many mghz of the DBM output. Another is hum pickup from magnetic fields. The more common used in radio circuits is a common base amp with an emitter current of .5ma which is about right for 50 ohms. This is the path that KK7B took and it offers both good voltage gain and a good match to the dbm. That is detailed in the book EMRFD, Experimental Design for RF Design, ARRL press. Worth its weight and more. Allison |
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Richard Seguin
This is very helpful, thank you
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On Mon, May 20, 2019 at 5:32 PM Pat Byers <ve3eur@...> wrote:
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Paul Martin
Rick Campbell KK7B designed a series of direct conversion radios with 50 ohm audio amps. See this link for details. May be some ideas you can use in yours. They work very well.
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Paul On May 20, 2019, at 3:48 PM, k7da <k7da@...> wrote: |
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One simple method you could use if maximum power transfer isn't a must
would be to shunt the output of the mixer with 50 ohms to ground. You could always make up audio gain. 73- Nick, WA5BDU On Mon, May 20, 2019 at 2:41 PM Richard Seguin <richard.seguin@...> wrote: Hello, |
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A common 50 Ohm input impedance amplifier design evaluation:
Wayne has used this design for RF amps in K3, KX3, KX2, and K2. And K2 post mixer amplifier. There are other examples out in the WWW. Some combine AC feedback with DC biasing. For this example, feedback is AC coupled for adjustment isolation. An observation, many of the examples suggest 50 Ohm in/out gain of about 15dB. Several different bias selections on my bench resulted with about 15dB gain. John KN5L |
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John KN5L writes:
A common 50 Ohm input impedance amplifier design evaluation:I have used common emitter amps similar to the above with feedback and a stiff partially bypassed emitter with good success. Another option I prefer is to use a common base (CB) amplifier. Characteristics are low-Z in, high-Z out with 10-15dB voltage gain depending on how it's biased. Thus, CB amplifiers work well as a low-to- high impedance converter with gain in both RF, IF and audio applications. I have used the CB amplifier on the output of a 50-ohm DBM mixer and 1.5K ohm output (collector resistor) for driving subsequent stages, and also to convert 50 ohm DBM output to 330 ohm crystal filter input. An emitter follower (EF) does the same in/out impedance conversion though with no gain. The EF and CB circuits seem to be highly overlooked in homebrew circuits where they can do the job well. At our HF frequencies, just about any old transistor would work. 2N2222s and a few resistors and caps are cheap. 72, Paul NA5N |
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ajparent1/kb1gmx
Paul,
For the case where the desired signal is audio and the DBM source is a Common base amp is the desired case. For a mixer to RF then other possible choice may have favor. The KK7B form is Common Base with a emitter current of about .5 ma. That will present a wide band load of about 50 ohms to the DBM and with a 3300ohm collector resistor would net a voltage gain of about 66. The noise figure of the amplifier is fairly small compared to feedback amps and the reverse isolation (S12) tends to be very good. Why .5ma... 26/ie-ma=Rin for an approximation. Common emitter needs to run at much higher currents for a solid 50 ohms in and the output must drive a 50 ohm load. The down side is with resistive feedback you get noise. IF this is a DCRX or Image rejecting DCRX that undesired added noise. Allison |
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ajparent1/kb1gmx writes:
Paul,The OP was primarily addressing audio, such as a DBM being used for a direct conversion receiver (or maybe the product detector in a superhet). Though, I have little experience building direct conversion receivers, mostly superhets. The KK7B form is Common Base with a emitter current of about .5 ma.Jim Kortge, K8IQY, also used common base IF amps in his original famous 2N2/40 rig here: On sheet 2, the Q5 CB amp matches the DBM to the XTAL filter impedance of 470 ohms, and the Q6 CB as an IF amplifier. Nice approach. On my common base amps (for RF/IF), I also set Ie at .5mA to set Rin at 50 ohms and usually use a 1.2-1.5K collector load to limit gain to around 12dB for stability. Or, a toroid in the collector (like 10-12TP:3TS) if driving a 50 ohm DBM. I've also had good luck using the internal output transformer in a DBM as the emitter source for the CB amp by swapping the IF and RF ports on the DBM, such as the ADE series (the transformer is the RF port). Instead of grounding the bottom of that RF port transformer winding, that goes to ground through a resistor to set the emitter current. One of those ADE-1 ground pins is the bottom end of the RF port transformer. the reverse isolation (S12) tends to be very good.Isolation is certainly another advantage of incorporating an active stage following the mixer. It can tame an unruly leaking LO if that is a problem. On the ADE-1 (and most DBMs), the LO-IF isolation is 50-55dB and the LO-RF isolation around 60-65 dB, another advantage of using the RF port for the IF output. Common emitter needs to run at much higher currents for a solid 50 ohmsOr use a MiniCircuits 50 ohm MMIC if you have 50-100 mA to spare! :-) A good discussion, Allison. A nice little tutorial and should give plenty of fodder to consider using either a common emitter or common base amplifier for matching to 50 ohms. There is no perfect solution, but lots of ways to skin a cat that works. 72, Paul NA5N |
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Hi Allison,
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Common Base Amp breadboard and measurements: Emitter resistor adjusted for 50 Ohm input Z. Can't get much closer to 0.5ma. KK7B form used 2.7k emitter resistor, Input Z was lower than 50 Ohm. Has a section describing V gain is not predictable. I wonder if input impedance is also component variable and requires tuning as done for this experiment? John KN5L On 5/22/19 9:52 PM, ajparent1/kb1gmx wrote:
The KK7B form is Common Base with a emitter current of about .5 ma. |
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Some additional comments:
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Added KK7B form emitter follower, no change in input Z. Input Z is a function of Vcc. Changing Vcc to 5V requires Emitter current ~= 0.7 ma for 50 Ohm input Z. Emitter load R = 1.36k A shipped application may benefit by using 5 or 8 volt three terminal regulator. Fun stuff! John KN5L On 5/23/19 6:47 AM, John KN5L wrote:
Hi Allison, |
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ajparent1/kb1gmx
Look at KK7B R2/miniR2/MicroR2 as his solution for power was a capacitance multiplier
at 12V. Higher voltage allows greater dynamic range. Looks like I wrote the wrong current down, though it should work there. At 12V a 1.8K emitter resistor and the correct base bias to get .52ma + base current (.02ma or so) will do fine. While common base is capable of very large voltage gain (basically ratio of source impedance to load impedance) practical values are lower. Typical device is 2n3904 or 4401 rather than a large device like 2n2222/2n2219 though they are quiet the HFE is generally lower especially at low currents. the optimal device i on that offers good gain (DC and AC) at under 1ma as well as being low noise. Its a circuit I've built and used . I also use a variation as a bidirectional amplifer to feed/and take audio from a DBM. In the feed the base is driven with audio and for RX (take) the base in grounded (shut diode switch+DC block cap) and the audio is taken from the collector with a 2.7K load. I built and used all three multiple times. Image reject direct conversion has a lot going for it in terms of sound quality. My 6M contest TRX is built around both teh RX and TX (miniR2 and T2). Its features and reason for the design was 1KW+ 13db antenna 800ft away usually aimed at me (that op runs over driven at that) so I needed a RX that would not crunch! I am able to run within about 5khz of him if I ignore his buckshot if he is on the opposing side band edge of me. Fun stuff indeed. Allison |
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Found some new ON Semi 2N3904 ordered a few months ago.
New schematic in: Using common part values. Very close to Zin = 50 Ohm. Moved output to mid Vcc, which increased gain some. Visible oscilloscope clipping at about 17 mVrms input, 5.48 VPP output. Assuming a 10-15 VDC field friendly rig with 8V regulated rail. John KN5L |
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Hi Allison,
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I was commenting about battery voltage range rather then current. Modern three cell LiPos sag down into 11V range. Some additional measurements at both 8 and 12 volt supplies. For 12 V operation, change common base emitter resistor to 4.7k for 50 Ohm input. Ve=2.369V Ie=0.50mA. All else unchanged. Gain does not change. Rough gain compression can be evaluated by adjusting oscilloscope input and output traces on top of each other. Increasing input until output trace sags with respect to input trace. Believe actual method is measuring in an out voltages to find 1dB gain compression point. Compression, using oscilloscope trace match described above is same for both 8V and 12V versions, at about 12-13 mVrms input. From this analysis, I see no benefit with the 12V version. KK7B form is featured in ARRL "Experimental Methods in RF Design," page 8.13. Discussion for low-noise AF amplifier is limited to it having no flaws and no additional design effort. One flaw is CB emitter resistor for 50 Ohm input Z. Most everything merits a revisit and possible improvement. I hope Richard, original poster, is benefiting from these breadboard measurements and study. Maybe reproducing these experiments for conformation. I found it to be quite educational! And fun to do. John KN5L On 5/23/19 6:34 PM, ajparent1/kb1gmx wrote:
Hi John, |
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Because it's fun to do, and may as well complete project before cleaning
up, plotted dynamic range. 2N3904 gain plot added. 1dB gain compression is -21dBm input. Measured down to about 1mV output. Below 1mV is both measurement and noise limited using a simple breadboard construction. Demonstrating about 70dB dynamic range, formalizing construction will increase measurable dynamic range. Measurement devices: Rigol DG1032 with 40dB pad input dBm values. Fluke 87V Hi-Res mode output voltage. John KN5L |
Nick Kennedy
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