Bob Cordell's Power amplifier book

[PB2]

Originally Posted by Bob Cordell

Hi Pete,

This is a really neat circuit technique. I wonder if there are any gotcha's? Looks like its set up for 2.5 mA. At lower frequencies, like 10 MHz, the 0.01 uF cap might want to go to 0.1. Using this circuit, we should be mindful that we oparate at a frequency where ac current gain is substantially smaller than LF current gain and preferably moderately greater than unity.

Cheers,
Bob

Yes, it is. You can see that it is match terminated with the 50 ohm resistor but I was thinking of leaving it out and just building it on a BNC to banana jack adapter so there is no need for a cable and termination. Also, this oscillator has a DC offset control, so with the 1K base resistor we get 1mA base bias current per volt of DC offset across that resistor eliminating the need for the negative supply. I'm also planning to use .1 uF for the bypass as you mentioned.
2.5 mA? Do you mean ball park? Ie would be about 1.9v/510 = 3.7 mA

[Bonsai]

I just read through the last few pages, and noted Ostrippers comments on the 340/350. I did not enjoy his success with these parts in the VAS stage - I almost always got HF parastic oscillation just as the amp was comming off its peak -ve output voltage (which is where I had it configured as a current source). I tried loads of things to try to cure it (base stoppers, changed Iq, played with loop comp etc etc). It was particularly bad when driving IRFP250/9240. Switched to BF469/470 (I got a pile from a friend at Philips) and the problem was cured. I suspect now that what was hapening was that Ccb was changing very dramatically through the voltage swing, and on the signal peaks its value was much higher. The 469/470's are HF video driver devices (now obsolete), and they never had this issue. I'd be interested to here any other feedback (BTW, after this I abondoned 'blameless' style amps and went FBS - but I guess the same issues would apply)

[padamiecki]

Originally Posted by Bonsai

...and went FBS ...

Hi Bonsai
please explain the shortcut fbs

[Bonsai]

Fully Balanced (input stage) with Symmetrical VAS = FBS

[Bob Cordell]

Originally Posted by PB2

Yes, it is. You can see that it is match terminated with the 50 ohm resistor but I was thinking of leaving it out and just building it on a BNC to banana jack adapter so there is no need for a cable and termination. Also, this oscillator has a DC offset control, so with the 1K base resistor we get 1mA base bias current per volt of DC offset across that resistor eliminating the need for the negative supply. I'm also planning to use .1 uF for the bypass as you mentioned.
2.5 mA? Do you mean ball park? Ie would be about 1.9v/510 = 3.7 mA

Hi Pete,

It would be interesting to simulate this circuit with a given transistor and calculate the ft as per the technique, then compare that to the ft as reported by LTspice.

Cheers,
Bob

[Bob Cordell]

I learned yesterday that TI is acquiring National Semiconductor. This indeed would seem to be a sad end of an era. National has always been one of the top, premier IC companies in linear ICs.

Perhaps this should be no surprize, given the layoffs that happened awhile ago at National of some top analog designers, including those involved with their high-end audio ICs. I hope that their legendary products will not be discontinued. I guess there is some hope there if one looks at what happened to BB when they were acquired by TI.

I guess the great National Semiconductor name will disappear after about a year. What a pity.

I know, I have a soft spot in my heart for the way things were.

Best,
Bob

[kenpeter]

TI's acquisition of Burr Brown didn't kill the products. Nor the name.
The volume of analog EVMs (including BB) I have to test each day
is becoming an extreme challenge. I'm sure its the same at all of TI's
CMs. Analog is huge at TI, and they are not going to mess this up.

My personal reaction was, "there go my weekends"...

[Macleod]

I work on RF. When Ti bought some years ago the small Chipcon we wonder what the product were going to be. In fact TI maintained the development center in Norway with the line of product written" Chipcon from TI". New products are an evolution of the product line with CCxxxx reference. in fact portfolio has increased an impressive way with TI as if additionnal resources have been injected. From a competitor point of view (I work for an old american semiconductor company that makes also mobile phones...) I can say the result is very good. Sometime I dream we would make such win-win deals...

[Bob Cordell]

Originally Posted by Bonsai

I just read through the last few pages, and noted Ostrippers comments on the 340/350. I did not enjoy his success with these parts in the VAS stage - I almost always got HF parastic oscillation just as the amp was comming off its peak -ve output voltage (which is where I had it configured as a current source). I tried loads of things to try to cure it (base stoppers, changed Iq, played with loop comp etc etc). It was particularly bad when driving IRFP250/9240. Switched to BF469/470 (I got a pile from a friend at Philips) and the problem was cured. I suspect now that what was hapening was that Ccb was changing very dramatically through the voltage swing, and on the signal peaks its value was much higher. The 469/470's are HF video driver devices (now obsolete), and they never had this issue. I'd be interested to here any other feedback (BTW, after this I abondoned 'blameless' style amps and went FBS - but I guess the same issues would apply)

Hi Bonsai,

Its true that the Ccb will increase as the VAS transistor has less Vce, but I'm not sure the 340/350 would be so much worse that it would provoke or allow oscillation in a circuit that was otherwise adequately stable. In other words, the overall circuit might have been on the edge even with the VAS transistors that worked. It sounds like the amp you had this trouble with was a MOSFET design. MOSFETs can be more prone to oscillation because they are faster, and if the gate stopper resistors (you mentioned base stoppers, so there may be some confusion on my part) are not large enough and/or other HF precautions for the MOSFETs are not in place, there could be interactions with the VAS current source transistor. Also, check what kind of current source you are using and whether it might have some possible Miller effects going on from its collector to base.

Also, try to determine if this is a global oscillation or a local oscillation. If you double the closed loop gain (e.g., double the value of the feedback resistor), does the oscillation go away? If it does, that suggests that the oscillation is in the global feedback loop. If it does not go away, it may be a local oscillation resulting from the interaction of the current source and the output stage. Also bear in mind that the drain-gate capacitance of the output MOSFETs gets quite large when you get near the rails and Vdg gets small.

Cheers,
Bob

[lineup]

MJE340/350 are good for higher voltage.
Otherwise I prefer BD139/140.
Think it is the higher gain that makes BD139/140 win over MJE340/350.

Many amplifiers have lower voltage than 80 Volt, the max of BD139/140
I have tried both options in SPICE amplifiers.
And BD139/140 gets substansially lower THD distortion than MJE340/350.