best topo for 25W class A into 1 ohm resistive load?

[Claude Abraham]

originally posted by John Curl
the reason for a base resistor is to add a POSITIVE resistance to the base of the transistor. This counteracts the NEGATIVE resistance that can appear at the base of a transistor follower with a cap load.

originally posted by traderbam
John,
I don't really understand how you get a "negative" base resistance as a result of a capacitive load. Can you explain?

Can you also explain why you should use a base resistor rather than an emitter resistor?

Greetings traderbam,

I assure you that John Curl has answered you correctly. The link below is an explanation of the cause of negative resistance in emitter followers that I posted recently in another thread. The post was a response to questions you raised, regarding emitter followers and feedback. A good reference text if you wish to see the full math derivation is "Pulse, Digital, & Switching Circuits", by Taub and Schilling, c 1965.

I've designed dozens of emitter followers, and I've seen the effect of negative resistance first hand. Local oscillation is the result. The oscillation is suppressed by adding a (positive) base resistor. Adding resistance in the emitter does not cancel the negative resistance seen at the base. The math (condensed) is in the link below. If clarification is needed, I'll do so. Best regards.

Claude



http://www.diyaudio.com/forums/showthread.php?s=&threadid=28764&perpage=10&pagenu mber=21

[Claude Abraham]

Sorry folks,

The link doesn't seem to be working. Here is an excerpt from my post from a few months ago.

"The hfe at high enough frequencies, becomes complex, having a phase lagging component. Instead of pure real, hfe may be 29 - j40 at high enough frequency. If the load is complex, resistive and capacitive, 50 - j60 for example, the complex impedance is reflected back to the base side can have a negative real part, i.e. -900 - j3800. This is essentially a negative resistance. The result is local instability, since the degenerative or negative feedback, becomes regenerative or positive feedback producing local oscillation. To stabilize, we must cancel the negative resistance by inserting some positive resistance in the base side. Any circuit designer who has used emitter followers (I've used dozens) knows this."

If I might add, the impedance seen at the input, or base, of an emitter follower (EF), due to emitter load Ze (output) is (1 + hfe)*Ze. Since Ze is resistive and capacitive, its imaginary part (reactance) is a negative number. Likewise, at a high enough frequency, hfe has a positive real part, and negative imaginary part. At a specific frequency, the product can have a negative real part, as shown above. This is a negative resistance. The voltage and current are displaced 180 degrees. Adding positive resistance in the base, neutralizes this negative value. I've used it with success. I always include this base resistor whenever I use an EF. I hope this helps. Best regards.

[thanh]

Hi! Linsource! Have you ever built your design?
I'm studying about servo circuit for Slone' topo.What opamp did you use in your design?
When I don't apply feedback , offset voltage is 18V .If I apply feedback ,it is about 256 micro ampere .I think this cause stabileity of my amp.

[LineSource]

thanh, I'm using AD8099 opamps for my servo only because I have some on hand. I see AD820AN and OP27 often used for servos.

Most of the semiconductors I ordered have come in and I have soldered up three different input-VAS stages and started tweaking and listening.
1) MAT02/MAT03 complementary differential..current source per diff.
2) JFET-JFET complementary differential..current source per diff
These two keep the servo out of the input signal path by tugging on the current source bias resistors as shown in my "design 2"

3) Stacked JFET-JFET_cascoded input complementary differential as shown in my "design 1"

All three sound different! My ears favor (3) until I hook up the servo opamp, then the sound becomes dull. Matching JFETs is very difficult and time comsuming. I had to purchase parts from two different vendors to get a close match.

I am using modest lab power supplies to drive the breadboard and can only get to 1-2 watts of class A. The breadboard VAS and output bias is very different from the final design.

I've also spend a few hours playing with a PCB layout

[AKSA]

Claude,

Many thanks; this is the best explanation for a base stopper on an EF I've ever read.

Cheers,

Hugh

[capslock]

Originally posted by Claude Abraham
Adding positive resistance in the base, neutralizes this negative value. I've used it with success. I always include this base resistor whenever I use an EF.

What I don't like about base stopper resistors is that they make the output stage gain even more load dependent (even if the relative nonlinearity might become better).

I have found that quite often a ferrite bead on the base lead will do as good a job as a 22R resistor in stopping local oscillations with highly capacitive loads. The advantage is that the bead acts near the frequency of oscillation and not also in the passband of the amp. From what I have read about those ferrites, they become pretty lossy at several MHz, so they would probably add more resistance than inductance.

[Easyamp]

I know this is way back but isn't 25 watts into a 4 or 8 ohm load the same as a 1 ohm load, it's still 25 watts???


Transistor turn-off is not an issue for Class A - thats one its points.

The question is 25W class A into 1R.

[LineSource]

easyamp,

The manuals for many commercial amps say that they are not stable into 1 ohm loads. Using simple P=I^2 R, 1 ohm requires 5 amps of bias and 8 ohms only 1.8 amps for 25 Class A watts.

If you are designing an amp for low impedance, high sensitivity speakers of > 100db/watt, you use can use lower power supply voltages which allows you to use transistor types and circuit topologies that higher voltage designs cannot, but you also have to execute a low noise design that is very linear for the first watt.

A good challenge for diyAudio experts.

[Easyamp]

I have no idea what you talking about, sorry.

[LineSource]

From listening to breadboard circuits:
1) A servo directly in the feedback path is audible. Dulls the sound. Reduces the dynamics. A servo that controls bias currents to null DC offset was used.
2) JFETS in the input and VAS have a more natural sound than bipolars.
3) A cascoded VAS changes the tonal balance of the mid-to-high frequencies, highs too strong. A +/- 12V pre-driver power supply supports a single FET VAS and a wide range of servo opamps.
4) For a <1 ohm ribbon, a 7-10V output power supply is very adequate and allows using small 8,200 @16V or 10,000uF @10V low impedance capacitors.
5) Sanken 2SA1216G and 2SC2992G are best sounding outputs.
6) Using a non-inductive resistor to pad the ribbon up to 1.5 ohms sweetens the sound...less edge on some CDs.
7) Ribbon direct drive is far superior to a transformer.... clarity, sound stage stability, dynamics..