Dual Gate MOSFET Investigations - Gain and AGC

Having determined some basic characteristics of the biasing of the BF998 dual gate MOSFET in a previous experiment, it was now time to look into the gain and AGC performance of the amplifier. A few changes were made to the original circuit to turn it into a proper RF amplifier.

Test Equipment

  • Power Meter: M3 Electronix FPM-1
  • Voltmeter: Fluke 8840A
  • Signal Generator: Tektronix SG503

Initial Test Conditions

The gate 1 voltage (VBIAS) was initially biased to 3.16 V, a level that was previously determined to give about 10 mA of drain current when gate 2 is biased to 9.2 V. The input signal was set to a frequency of approximately 28.1 MHz, to give an idea of the amplifier performance in the upper HF bands. The output power of the signal generator was set to -30.0 dBm into a 50 Ω resistive load. This gave me enough power to make a good measurement with the FPM-1 while avoiding the problem of gain compression. All gain measurements are based off of this amplifier input power (in other words, the amplifier gain described in this report is the transducer gain).

The Circuit

The DC biasing of the circuit is virtually identical to the final configuration determined in the first experiment. However, there have been some changes in regard to the input and output circuitry. First of all, the gate 1 bias is now fed through a 2.2 kΩ resistor which is bypassed to AC ground with a 100 nF capacitor. This sets the input impedance to 2.2 kΩ. Values around 2 kΩ seem to be fairly common in the literature, apparently because of the noise figure benefits. I would like to investigate this further in a later experiment, but for now we'll go with the wisdom of others. A typical L-network was placed on the input to transform the 50 Ω amplifier input impedance to the 2.2 kΩ impedance that gate 1 wants to see. The drain inductor was replaced with a 10:2 ratio transformer to give the drain a load of 1.25 kΩ to work into when a 50 Ω load is placed on the amplifier output. Again, this is another area where I decided to go with the wisdom of others. This drain load values seems reasonable based on other FET amplifiers I've used, but it might also be an area worth investigating later.

Dual Gate MOSFET Gain

Results

Under the initial conditions described above, I measured an output power of -6.1 dBm, which indicates a transducer gain of 23.9 dB. This seems like a reasonable and believeable amount of gain from a single amplifier given the biasing levels established. I decided to vary VBIAS a bit to determine the point of maximum gain for the amplifier. At a gate 1 voltage of 3.43 V, I measured -5.9 dBm of output power, or a gain of 24.1 dB. There is a slight amount of difference between the two voltages, but not enough to be significant. It seems that the initial estimate worked fairly well.

AGC Characteristics

Next, the circuit was modified slightly to examine the AGC characteristics of the BF998. Both the source and gate 1 were biased to approximately 3 V using a blue LED. This biasing method is very convenient, simple, and stable, even if it may not bias gate 1 to its ideal point. This reduced the drain current to 6.6 mA, which would mean a slightly lower maximum gain, but also would be a more power-efficient way to run the amp. I could have used two red or green diodes in series, or a string of small-signal diodes as seen in the Hybrid Cascode amplifier, but the blue LED uses the least parts (and is pretty to boot). The fixed voltage divider bias was removed from gate 2, and in its place a variable AGC voltage (VAGC) was applied. The same -30.0 dBm input power was used, and the output power was measured at different settings of VAGC.

Dual Gate MOSFET AGC

Results

As you can see in the table and chart below, there is a large AGC voltage range with very little gain variation, then a sharp knee where there is a steep slope of gain reduction. The knee occurs at an AGC voltage of about 3 V. Between 2 V and 3 V is the largest gain variation (about 40 dB). This AGC response curve actually appears to agree fairly well with the curve published in the Philips RF Manual 3rd Edition Appendix for the BF998. The AGC range of approximately 50 dB also seems in line with the data published by NXP. It does look plausible that two of these amplifiers cascaded together could provide nearly 100 dB of gain reduction (another experiment idea for later).

VAGC POUT GT
2 V -55.7 dBm -25.7 dB
2.25 V -52.8 dBm -22.8 dB
2.5 V -37.2 dBm -7.2 dB
2.75 V -23.3 dBm 6.7 dB
3 V -16.0 dBm 14.0 dB
4 V -12.6 dBm 17.4 dB
5 V -11.2 dBm 18.8 dB
6 V -10.4 dBm 19.6 dB
7 V -9.9 dBm 20.1 dB
8 V -9.5 dBm 20.5 dB
9 V -9.1 dBm 20.9 dB
10 V -9.0 dBm 21.0 dB

Dual Gate MOSFET AGC Graph

Next, I intend to build a return loss bridge (finally!) and get some measurements on this amplifier. I also need to look into what it will take to measure noise figure, and get started on that test rig as well.

pQRP LC Meter Complete

A quick update to show that I have completed the pQRP LC meter and placed it in a really nice enclsoure. I finally made it down to Norvac to pick up a few enclosures and found that the Hammond 1590CBU was just about a perfect fit for the meter. It's made of die-cast aluminum and has a nice blue paint job. It wasn't cheap, but I feel that this instrument is worthy of a nice enclosure that will protect it for years.

pQRP LC Meter - Internal

pQRP LC Meter - Complete

The LCD and PCB fit just about perfectly in the lid, along with the binding posts and switches. The coaxial DC power connector was placed in the body of the enclosure itself, at the top of the meter as it faces your. Some labels from the label maker finished it off quite nicely. Overall, I'm extremely impressed with this meter. Perhaps NB6M will see fit to produce another kit run to allow more US hams to get in on this great piece of test equipment.

Dual Gate MOSFET Investigations - Biasing

I've had a large collection of BF998 dual gate MOSFETs sitting around in the junkbox for quite a while now (acquired from KE6F), but I've never really known what to do with them. I feel like I own a fairly decent library of QRP/homebrewing literature, but there seems to be very little detailed information on the theory of operation of the dual gate MOSFET. Most of sparse circuit information on the device seems to assume that you already know the basic theory behind it.

I decided that the best way to learn more about the workings of the dual gate MOSFET was to start building circuits and taking measurements. This post is intended to be the first in a series where I will examine different aspects of dual gate MOSFET circuits.

I started by consulting the BF998 Datasheet, as well as additional documentation provided by the manufacturer, NXP. I decided to base my first circuit off of the recommended biasing in the NXP documentation. Below is the schematic for the first setup that I used to examine the bias characteristics of the BF998. A constant voltage (VAGC) was applied to G2 for this test. The datasheet recommended a nominal AGC voltage of 9 V, so I tried to get a voltage on G2 somewhere near that value. I applied a variable bias voltage (VBIAS) to G1 and measured the voltage drops across R6 and R7 in order to calculate the currents in each resistor. The difference in the currents in R6 and R7 indicated the drain current (ID).

Schematic for Dual Gate MOSFET Bias Experiment

VBIAS VR6 VR7 ID
0 V 2.19 V 9.94 V 0 uA
2 V 2.43 V 9.70 V 890 uA
4 V 3.97 V 8.15 V 6.60 mA
5 V 4.78 V 7.35 V 9.58 mA
6 V 5.60 V 6.53 V 12.62 mA
8 V 7.20 V 4.92 V 18.54 mA

This data gives us a starting point to see the effect of VG1 on ID. One feature to note about this biasing scheme is the voltage divider bias applied to the source. The literature states that this is used to bring the source to a higher potential than would be possible using a simple low-value source resistor. The rationale for this is to allow the AGC voltage on G2 to be able to swing lower than the source voltage, which will give a much wider AGC range than a biasing scheme with a single source resistor. (This negative voltage in relation to the source is necessary because the BF998 is a depletion-mode FET, similar to your common N-channel JFET. If you look at the datasheet, you see that the pinchoff voltage is negative). I'll touch on this a bit more shortly.

What is interesting is how this biasing scheme differs from the one encountered in most of the QRP literature. Next, I tried to duplicate the biasing scheme scheme used in the W7ZOI 50 MHz preamp. However, I was unsuccessful in getting any significant amount of drain current with G1 grounded (not nearly the amount that W7ZOI was reporting, even when using the same source resistor values that he reported). I could get much more drain current to flow when I applied bias voltage to G1 as I did in the previous experiment. This puzzled me, so I decided to set it aside for now.

A bit more searching on the net lead me to a circuit in another radio; the Electroluminescent Receiver. Dual gate MOSFETs are used liberally in this radio. It turns out the dual gate MOSFET circuit in this receiver derives its lineage from a QRP classic; the Progressive Communication Receiver (ARRL members only). The unique bit about the amplifiers in these receivers is the use of a diode or LED in the source leg to provide biasing. The forward voltage of the diode(s) or LED(s) raises up the source voltage to allow that AGC swing that was mentioned earlier. This is an alternate (and I believe a simpler) way to do the same thing as the voltage divider biasing used previously.

Armed with this information, I changed the biasing to try a LED in the source leg. A green LED was used, since they have a forward voltage of approximately 2 V. I tried two different values of R6, and as before I applied various bias voltages to G1 to see the effects on drain current.

Schematic of dual gate MOSFET bias experiment

R6=330 Ω

VBIAS VR6 ID
0 V 0 V 0 uA
2 V 512 mV 1.55 mA
3 V 1.25 V 3.79 mA
4 V 2.03 V 6.15 mA
5 V 2.82 V 8.55 mA
6 V 3.63 V 11.00 mA
7 V 4.42 V 13.39 mA
8 V 5.21 V 15.79 mA
9 V 5.70 V 17.27 mA
10 V 5.83 V 17.67 mA

These results seem pretty good, although it looks like I was running into the limit of increasing the drain current somewhere around 9 to 10 volts. I suspect that's because the voltage on G1 was approaching the voltage on G2. However, I may be wrong about that, so feel free to correct me.

Next, I decreaed the source resistor to 100 Ω, a value commonly seen in other dual gate MOSFET amplifiers.

R6=100 Ω

VBIAS VR6 ID
0 V 0 V 0 uA
2 V 355 mV 3.55 mA
3 V 912 mV 9.12 mA
4 V 1.52 V 15.2 mA
5 V 2.15 V 21.5 mA
6 V 2.79 V 27.9 mA

Since the maximum drain current for the device is listed as 30 mA, I stopped at 6 V of bias on G1. A bit of tweaking of the bias voltage showed me that I could get a drain current of 10 mA with a G1 voltage of 3.16 V. Since the manufacturer documentation lists a nominal drain current of 10 mA, I decided to stick with this value for now. When it comes time to test IMD, I may want to change this a bit, but for now it's a good starting place.

The plan is now to use the LED biasing scheme when I start investigating the RF characteristics of the device. I suspect a bit more tweaking will be done, but I feel that I have a better grasp of the basic biasing of the circuit. I may look at taking the G1 bias off of the top of D1, or I might use a voltage divider...that's still to be decided by future experiments.

pQRP LC Meter Kit

Wayne McFee, NB6M has created a custom PCB for his variation of the MARC LC Meter, along with providing a kit of parts. I just received my kit yesterday and was very eager to build the thing, since it's one vital piece of test gear that I don't yet have.

Stuffing the PCB according to Wayne's instructions was quick and easy. There were a few minor hitches to watch out for, but nothing big as long as you take the time to read the documentation. It only took me about 45 minutes to get the on-board components installed.

pQRP LC Meter - Halfway There

One nice thing about the kit is that the PCB fits the exact dimensions of the white-on-blue LCD display that is provided. The mounting holes line up quite nicely, which allows you to use standoffs to secure the display to the main PCB. A few external switches and connectors are wired to the PCB to finish the electrical construction of the meter.

pQRP LC Meter - Up and Running

The smoke test passed successfully and the meter read a capacitor and an inductor pretty close to the nominal values, satifying me that everything was working correctly. I know that the leads from the DPDT switch to the DUT terminals should be shorter to minimise stray inductance, but the accuracy was reasonably good with the setup that I have above. I'll probably end up switching out those binding posts anyway, since I'm not too fond of them. I've got a supply of 5-way binding posts from Jameco that I like to use, so I'll substitute those with shorter leads.

All that is left to do is mount the meter in an enclosure, which I still have to procure. I'll probably make a trip down to the local Radio Shack later on to get a project box. (I can't believe that I'm actually not ashamed to shop at this RS). Kudos to Wayne for an excellent and very useful kit. I know that this one will be getting a good workout on my bench.

What's On Your Bench?

Terry, WA0ITP asks on qrp-l.org:

With a low Solar Flux and a double digit A index, it's time
to hunker down and melt solder, snort rosin, and burn
fingers.

I've got a half dozen unbuilt kits, an oscillator, single
ended mixer, and an audio amp all destined to be a DC
receiver (hopefully).  An 80M amp nearly installed in a CR
enclosure.  Oh, and a buncha Dayton RCA connectors for a
straight key/keyer distribution box, err tin..

So What's On Your Bench?

The answer at NT7S is: way too much stuff. On the kit side, I have two from NB6M of the pQRP group: the TinEar receiver and his version of the MARC L/C Meter. I recently ordered the N3ZI Digital Dial, and have that about half assembled. I plan on pairing this up with my beta version of the Willamette when I get a chance to put it in an enclosure. I've also got a big pile of the W8DIZ RF Toolkit modules ready to patch together into a receiver. The plan is to pair this stuff with the HYCAS IF amp.

On the homebrew side, I've got a whole bunch of irons in the fire. I've really been taken with AVR programming lately, so I've been developing some different applications for these uCs. The development environment is Ubuntu Hardy Heron, using gedit and the avr-gcc tools, and the Adafruit USBtinyISP programmer. The big sooper sekrit project here is an in-line QRP SWR meter, which is rumored may make an appearance as a kit if I can learn how to use Eagle CAD. Many other AVR-related ideas are also on the back burner. Believe it or not, I've also been doing some analog RF stuff. I have a dual-gate MOSFET mixer (based on the BF998) that I've been tweaking for possible use in a simple receiver. I think the plan is to put it on hold for a while, in order to build up some more test gear (such as a return loss bridge, noise figure test rig, etc.) so I can quantify the performance better than any other circuit I have yet developed.

So as you can see, I've got way too much stuff on my hands. Even if I could retire really, really early, I would have enough projects to keep me busy for very long time.