|Using a base-collector strapped transistor as AM detector, sharper knee for better weak signal demodulation|
The emitter on such an NPN transistor would be the cathode, and the base tied to the collector is the anode Here I used a 2SC2785 salvaged from a VCR or such. If the transistor is a PNP, its emitter would be the anode, and its base tied to its collector the cathode. Try various transistors. I found that 680K seems to hit the sweet spot in this radio, but it's not a critical adjustment on a trimpot (which you can use to quickly find the value you'd want in a fixed resistor). I had thought that using a transistor this way was the mark of a sloppy company (did they run out of diodes?), but I got the best weak station detection performance with this. Oh, check strong stations too, which worked well too.
Turns out I needed a bit more sensitivity in the AVC detector diode, so I used a germanium
transistor with its base and collector strapped together. I found that physically larger transistors
have lower base-emitter drops (the 2N1304 has 0.14V with the base and collector tied together) than smaller germanium transistors. But be aware that most larger germanium
transistors may not be up to switching fast enough to act as a detector. I used the 2N1304,
its data sheet says it can do a few MHz. I now get less or no distortion on very strong stations,
as the AVC now kicks in with less "delay" ("AVC delay" means the AVC needs a certain voltage
amplitude before the AVC starts reducing the stage gain); it's not time delay.
The big transistor with heat shrink over it is the large germanium transistor. Its case is tied to its collector, and the heat shrink is to prevent shorts.
Some simulations of a silicon diode, a base-collector strapped silicon transistor,
and such a transistor with a bias current.
I'm not sure how much, if any, knee can be below 0V here. Conduction on the other side of 0V would probably mess up the detection. IIRC, the FCC requires that 10% be the minimium amplitude of an AM station's carrier, which should permit some knee curvature of stations that aren't too weak (else signal to noise would make them unlistenable anyway).
Here the usual AVC circuit in the AM section of this radio gets an assist from a JFET operating as a voltage controlled resistor. This is in the triode or ohmic region (at and near 0V and 0mA current, the blue region in the figure below) where the FET can operate as a voltage-controlled resistor. The triode or ohmic region in an FET is sometimes known as the linear region. The JFET operating as a voltage-controlled resistor works in this region. If we extend the VDS voltage range to include slightly negative voltages for a particular gate-t-source voltage, we see that the there is still a resistive effect. Preferably, there is no DC voltage across the JFETís drain and source terminals in the voltage controlled resistance mode. This JFET's source and drain is connected across the entire LC circuit of the first IF transformer, which has no significant DC across it. The source and the drain both have IF frequency energy on them, out of phase with each other. The LC circuit has a tap near the center of the winding, tied to an RF ground (which is why the drain and source has out of phase IF energy) The gate is roughly at the midpoint of the resistance, which should reduce the effect of the gate capacitance on the IF.
When the gate voltage VGS here is dropped below around -4V, the JFET (a J211) becomes near infinite resistance. At around -2.7V, the JFET becomes a resistance around 1500 ohms. This variable resistance across the IF transformer LC circuit attenuates the signal strength, and also as a side benefit, will make that LC circuit have wider bandwidth. Which will allow higher audio frequencies from the strong station to be heard, higher fidelity (also better fidelity because the attenuation keeps the strong station from overdriving and distorting in the rest of the IF strip). Weaker stations will have narrower passbands, avoiding "monkey chatter" from immediately adjacent weak stations.
The controlling voltage is acquired from the AVC line via an inverting transistor (a 2N2222). This transistor level shifts and amplifies the AVC voltage range. Weak signals create a higher AVC voltage, and the transistor goes into more conductance, drawing, via the collector resistor, its collector voltage more negative. Strong signals will produce lower AVC voltages, and the transistor goes nearly into cutoff, and the collector approaches the positive supply rail. Be sure to use a bypass cap between this collector and the B+ line, to avoid ripple and such from "modulating" the voltage controlled resistances of the JFETs. Which would distort the detected audio. You need to size this cap to smooth out ripple, but still allow the circuit to respond to the AVC changing due to tuning in stations. I used a trimpot to get a sweet spot of an emitter resistor value, setting the gain of this transistor (a value that has the JFET do nothing on weak stations, but attenuates strong stations to minimize distortion), then I used a fixed resistor of this value in the finished circuit. Not a great way to do circuit design, as variations in the transistors and JFETs could throw this off in production. The IF transformer's LC circuit is also at the positive supply rail. From the JFET's point of view, (its gate is connected to the transistor collector) its gate going more negative makes the JFET resistance go higher. The gate getting less negative makes it lower resistance, attenuating the strong station's signal. As the AVC circuit is a feedback loop, the JFET's resistance will settle to a value after this loop does its action.
A second JFET
on the IF transformer between the first and second IF stages is also installed. You don't want to
put it at the last IF transformer, as all you'd do is attenuate distorted signal. A third JFET
is also used on the antenna LC circuit (I removed a 1.5pF cap there, as the JFET has some
internal capacitance, a few pF's, inside it). Then I retweaked the antenna trimmer cap.
Above right shows an exaggerated relation of passbands (how wide some value of X dB down) to radio station signal strngths. The passbands really vary by a factor of two at most. Attenuation is related to AVC action, in that weak stations are amplified more than strong stations in regular AVC circuits, and here there is actual attenuation being done as well (which via the voltage controlled resistive JFETs changes the passband widths of the various IF transformers and the antenna LC circuit).
Another radio, this one AM:
Here is a single JFET connected across the 2nd IF transformer T2. The IF amp transistor TR2 also acts like the 2N2222 in the above AM/FM radios (as TR2's base gets biased by the AVC line), and the JFET control signal is picked off a bypassed resistor R10, was 1K, now 1.5K (to increase the control voltage range). The full LC circuit of T2 is bypassed by C18 and biased by voltage divider R6 and R7. There was a diode between T2 and R10, but I removed it before I added the JFET. The voltage divider was reworked (by changing R6 to 180 ohms) to have the JFET go almost into infinite resistance when the radio is tuned to an empty spot on the dial. And to drop resistance value on strong signals. This then widens the IF bandwidth and reduces the IF strip's gain.
Another AM radio, an RCA RZA202B, with the JFET mod. One JFET across the antenna LC circuit L1, and another JFET across the first IF LC circuit T1. Driven by a similar buffer transistor that uses the AVC line voltage and converts to to the JFETs' gate control.
This circuit samples the voltage of the IF signal on a collector of a transistor in the IF amp. And is independent of the AVC circuit. The diode D2 "detects" the amplitude of the IF signal (the diode pointed so that more IF amplitude makes the bias on Q1 increase, thus making the LED light brighter), either 455KHz or 10.7MHz in an AM/FM set I put this into. Resistors R2 and R3 were a single 470K trimpot I used to adjust the bias on Q1 (the wiper connects to the base) to get the LED on Q1's collector to light or dim as I tune across the AM or the FM dial. Measured the trimpot, and then used 39K for R3 and 560K for R2. Your mileage will vary, depending on the diode you use for IF level detector D2 and transistor for Q1. R1's resistance was chosen to be a negligible load on the signal getting amplified by the IF stages of the radio. That then drives the selection of R2 and R3's resistance. The LED is a modern high efficiency one that doesn't require much current to light up bright. It came from a recently made Xmas light string. Thus Q1 doesn't need much biasing from the "detector" circuit to delver enough current to light the LED.
I wanted to avoid loading the AVC circuit itself. This above circuit shows what the AVC has already done to the signal. Loading the AVC circuit directly would tend to make the AVC circuit less effective in controlling the IF gain.
The FM section of the radio I put this in doesn't really have AVC, as you want the FM signal to get clipped before the discriminator detector anyway. I can see a station with full quieting still be intermediate signal strength, and stronger stations are of course full quieting. This above circuit shows the varying levels of station strength beyond full quieting (useful for antenna aiming). I tapped the 2nd to last IF stage (TR7's collector in the schematic in the JFET section of this page), where clipping hasn't fully kicked in yet, but full clipping does happen in the final IF stage.
You know the old loose bipolar NPN or PNP transistor test method of checking the PN junctions (with
your DVM in diode mode), like
from the base to the emitter, and then base to collector. You can figure out if it's an NPN or a PNP
transistor (compare to a diode). And additionally, you can see some transistor activity by doing this
following test: First, having
one test lead on the emitter and the other on the base (but not the collector), in the direction you see a diode drop voltage,
typically you'd see about 0.68V on a silicon transistor. Now while keeping the same test lead on the emitter,
touch at the same time with the other lead both the base and the collector. You should see a lower
voltage like 0.59V (around 85% lower). This test can also tell you, with surface mount devices, if you
really have a transistor, or a device with two diodes, one with a common anode, or a common cathode on what would
look like the base pin.