Lets talk about amp biasing and building. Bring in the experts.

[025]

Ok. So im trying to learn more about amps and amp building.
A see that a lot of you guys are able to look at amplifier and spot whether it's class C or AB just by looking at it.
I thought the diode across the power in had something to do with it?
But ive heard of bias resistors. I guess they would be on the base of the transistors?
I have a few mrf 455 2pill amps im trying to understand. Theres a hooker 100 has a weird switch says pep and rms..otherwise high or low from what i can tell. And just adds a resistor.
I have the classic palomar tx100 with weird tx100 pills. A silver eagle 200 and a palomar 225. All use the mrf455 except the tx100 and the hooker is stud mount.
My idea is that if I can't understand how is simple little amplifiers work that I don't have the knowledge to figure out how to build one successfully either.

 

[Shadetree Mechanic]

This guy shows what he does without a lot of bs.

 

[025]

This guy shows what he does without a lot of bs.

Unfortunately the video after 50 mins of just showing him do assembly trying to make it look pretty gave no info behind anything.
I am looking for how things work. Not how to build amp. How to design a bias circuit. How to change bias on other amps. That kinda thing.
Thanks for looking out though.

 

[AudioShockwav]

There is a sticky at the top of the Amp section about Bias, lots of good info.

73
Jeff

 

[025]

There is a sticky at the top of the Amp section about Bias, lots of good info.

73
Jeff

Thanks! I'll check it out for sure!

 

[Shadetree Mechanic]

Here is a document that deals mostly with tubes, but it will help explain the class of bias.

 

[Shadetree Mechanic]

More bias.

 

[AudioShockwav]

Here is a straightforward article about bias with some good info as well.
It is a good read to help understand bias in rf amps.

http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm

73
Jeff

 

[025]

Here is a document that deals mostly with tubes, but it will help explain the class of bias.

I have some tube amplifiers..sb220, hl2200 etc.

 

[025]

Here is a document that deals mostly with tubes, but it will help explain the class of bias.

I think i may have read that a few years ago when i was redoing my 220

 

[025]

Here is a straightforward article about bias with some good info as well.
It is a good read to help understand bias in rf amps.

http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm

73
Jeff

This was spot on. Thats what i wanted to know. The design is in ham pa's but ive never seen a cb amp with that kinda bias. What is it they use in the typical mrf455 2 pill amps?

 

[Handy Andy]

Best to learn this process by playing fair.

Not ALL Bias designs use that at the link...

Similar, but not exact - and can have disastrous results...

​

Fig a = typical DRIVER bias found in Uniden and Cobra radios...

Fig b = typical FINAL bias found in Uniden and Cobra radios ..

The differences is subtle - but are major when it comes to drive currents.

To help this along, Many ask why even use the 10 ohm resistor - you know, they comment on that "10-ohm Smoke?"

Well, Class C or not, Transistors themselves when exposed to AC or RF - any form of signal - tend to RECTIFY that signal at the B/E junction - so to completely understand the aspects of BIAS, you also need to respect the efforts of the Bipolar is rectifying part of the signal - so the line SKEWS DC bias UPWARDS in proportion to the input level of signal - the greater the signal level - the stronger that bias component gets and works against the BIAS regulation circuit - no matter which way you try to compensate...

So to ease that dilemma - I often use a Resistor subbed in with the RFC choke to keep RF out of the Bias circuit and also provide a weak current offset to keep RF to the Base and allow the BIAS to push current thru the Resistor / RFC combo as a control current and thereby when the RECTFIED bias voltage at the Base of the transistor creeps up - it will also be impeded by this same circuit to help reduce the current flow BACK into the BIAS control circuit - raising the level of current and the voltage presence.

Note the highlighted "C1" - Larger = Worse reference. When you rectify the RF, too high of a value in C1 can make the voltage presence and RF still feeding back thru into the Bias control - higher than what would be safe even when the transistor is turned off.

• - the rise and fall lag of the bias with C1 values too high, can leave the transistor latched on for a greater period of time that the device can easily dissipate - running the risk of latching and self destruct
• - as seen/shown in the DRIVER bias change of the additional 180 ohm as a voltage and current (power) divider to lessen drive current and the voltage rise that follows.

It is in the Selection of values that also mitigate the excessive drive voltage that can damage the Bipolar - so the effort here is to operate and use values of both R and C in the above circuits to reduce RF - but allow DC bias to keep flowing into and out of the BIAS circuit and also maintain a SOA for the Base of the Transistor.

 

[025]

Best to learn this process by playing fair.

Not ALL Bias designs use that at the link...

Similar, but not exact - and can have disastrous results...

View attachment 41873​

Fig a = typical DRIVER bias found in Uniden and Cobra radios...

Fig b = typical FINAL bias found in Uniden and Cobra radios ..

The differences is subtle - but are major when it comes to drive currents.

To help this along, Many ask why even use the 10 ohm resistor - you know, they comment on that "10-ohm Smoke?"

Well, Class C or not, Transistors themselves when exposed to AC or RF - any form of signal - tend to RECTIFY that signal at the B/E junction - so to completely understand the aspects of BIAS, you also need to respect the efforts of the Bipolar is rectifying part of the signal - so the line SKEWS DC bias UPWARDS in proportion to the input level of signal - the greater the signal level - the stronger that bias component gets and works against the BIAS regulation circuit - no matter which way you try to compensate...

So to ease that dilemma - I often use a Resistor subbed in with the RFC choke to keep RF out of the Bias circuit and also provide a weak current offset to keep RF to the Base and allow the BIAS to push current thru the Resistor / RFC combo as a control current and thereby when the RECTFIED bias voltage at the Base of the transistor creeps up - it will also be impeded by this same circuit to help reduce the current flow BACK into the BIAS control circuit - raising the level of current and the voltage presence.

Note the highlighted "C1" - Larger = Worse reference. When you rectify the RF, too high of a value in C1 can make the voltage presence and RF still feeding back thru into the Bias control - higher than what would be safe even when the transistor is turned off.

• - the rise and fall lag of the bias with C1 values too high, can leave the transistor latched on for a greater period of time that the device can easily dissipate - running the risk of latching and self destruct
• - as seen/shown in the DRIVER bias change of the additional 180 ohm as a voltage and current (power) divider to lessen drive current and the voltage rise that follows.

It is in the Selection of values that also mitigate the excessive drive voltage that can damage the Bipolar - so the effort here is to operate and use values of both R and C in the above circuits to reduce RF - but allow DC bias to keep flowing into and out of the BIAS circuit and also maintain a SOA for the Base of the Transistor.

Excellent info

 

[025]

It looks like all these so-called 2 pill amps are bad

 

[025]

So i guess this one is questionable?