Why not use Darlington transistors?

[Tallman]

They are plentiful and inexpensive. I had a mobile audio amp way back when and it put out 90 watts per channel. Truly thunderous inside of a 1965 Dodge Carryall. Other than the seats there was no upholstery, carpet, or other sound deadening surfaces.
The point is that the environment was less than ideal for electronics. That amp worked for over ten years with no problems and it had Motorola MJE6040 Darlington transistors.(I think that's the part number.)
With the much higher gain available over standard bipolar types it seems to me that it should be fairly easy to build an amp using them.
Has anybody tried this? If you have, do you have schematics, pcb layouts, etc.

 

[Captain Kilowatt]

With very high gain you have issues with stability. Darlingtons are generally better suited for switching applications where a very low control voltage is present. Switching times may be an issue at RF as well.

 

[Tallman]

With very high gain you have issues with stability. Darlingtons are generally better suited for switching applications where a very low control voltage is present. Switching times may be an issue at RF as well.

The stability issues could be handled with a small amount of negative feedback and the rise and fall times are better than most FET's.
Base bias issues are some what simple to deal with a caps, resistors, and diodes. There was an amp I had at one time had a portion of the circuits called "Bootstrap" which adjusted the gain and base bias with negative feedback.

 

[Captain Kilowatt]

What was this amp used for? Audio or RF?

 

[Tallman]

The bootstrap unit was for RF. It was a Motorola experimental unit built for a proof of concept for a contract.

 

[Tallman]

What was this amp used for? Audio or RF?

I thought the "Bootstrap" thing was funny. I worked in the oil field and we had these PDP-11 computers and we had to load in the bootstrap before we could download the programs from the Texas Instruments terminals contained on audio tape cassettes. What you had to do was flip a bunch of these switches in order and then hold a switch and then flip another one. Some times 6 to 7 pages of settings and one wrong position and you had to start over from the beginning.

 

[wavrider]

I thought the "Bootstrap" thing was funny. I worked in the oil field and we had these PDP-11 computers and we had to load in the bootstrap before we could download the programs from the Texas Instruments terminals contained on audio tape cassettes. What you had to do was flip a bunch of these switches in order and then hold a switch and then flip another one. Some times 6 to 7 pages of settings and one wrong position and you had to start over from the beginning.
View attachment 17902

Sounds like the encryption codes we used to have to load in the military aircraft, so many damn different settings and all had to be exact,

Now it is all probably done digitally

 

[Tallman]

Now it is all probably done digitally

Actually, I think it is done with EEPROMs or Eproms these days. My worst nightmare was the old YUK-7 computers that were used on US Navy war ships for weapons fire control and whatever else they could computer control.

 

[Onelasttime]

Like other have said not so hot in RF. Ever hear the hash from a cheap switching power supply??? I also do not think anyone makes any Darlingtons for RF purposes but I have not researched that I am pulling that out of my rear so treat it as such.

Also keep in mind that high current devices in audio compared to high current devices in RF are world apart. In audio the hash is not an issue because it is not at the freq. of the audio and it is easy to filter out if you even need to specifically filter it. I Pheonix Gold was one of my FAVORITE high current car audio amplifiers when I did that in High School. They where huge on Darlington transistors. Often at trade shows and competition they would be sitting at a both cutting pop cans in half with speaker wire and a nail with the output from their amps. The idea being if the amp lived happily at a short it would be fine with any reactive load you put on it with speaker like nominal 1/2 ohm load etc....

Back when discrete components ruled the earth and red meat and smoking where still popular and politically correct as long as you biased the transistors right and had lots of cap's and huge amount of steel and copper you could get almost any sound you wanted. That was what made Carver such a hit in the home audio world.

In RF we especialy mobile not home based operating voltage, gain at given voltage and gain at the freq. we want to operate at are all super important. Then we also need to look at imd and how many watts the device can dissipate. Car audio is no where near as demanding. Oh and lets not forgot about swr and what it can handle. Fet's that produce worth while gain at the 12-16V range at 27Mhz tend to be not so durable and need a very low VSWR. They can have very very clean output but the output is not as ideal for rf as a bipolar. When we move to LDMOS operating voltages go up and cost goes up and at 12V they have almost no gain at all. So if you want to run a device that needs 50V-200V you have to have a way of steeping up the 12V and that add's weight, complexity, size,heat and reduces safety to the occupants of the car and to the public. I mean it is not as bad as putting a 500Z tube amp in the trunk but you are moving in that direction. That is why low voltage bipolars have been the longterm Princess if you will.

 

[Onelasttime]

I thought the "Bootstrap" thing was funny. I worked in the oil field and we had these PDP-11 computers and we had to load in the bootstrap before we could download the programs from the Texas Instruments terminals contained on audio tape cassettes. What you had to do was flip a bunch of these switches in order and then hold a switch and then flip another one. Some times 6 to 7 pages of settings and one wrong position and you had to start over from the beginning.
View attachment 17902

I remember when PC's had crystals on the board! I also remember needing a math co-processor sold separately! My first computer taught me Fortran/COBOL/Basic/ and about 10 other's. When I entered college they where still teaching ASL400. I can remember when hard drives sounded like a they where making popcorn! My first computer had a tape drive. LOL