Bob Cordell Interview: BJT vs. MOSFET - diyAudio
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Old 14th November 2006, 02:10 AM   #1
AJT is offline AJT  Philippines
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Hi Bob,

i know that J Curl and Leach favor bipolar output transistors as opposed to you favoring mosfets, care to elaborate on this?

again, your circuits seem to be single ended at input and VAS stages, am i correct or do you have any design that is fully complimetary from input to output?

thanks..
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Old 14th November 2006, 12:14 PM   #2
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Default Why I prefer power MOSFETs

Quote:
Originally posted by Tony
Hi Bob,

i know that J Curl and Leach favor bipolar output transistors as opposed to you favoring mosfets, care to elaborate on this?

again, your circuits seem to be single ended at input and VAS stages, am i correct or do you have any design that is fully complimetary from input to output?

thanks..

Hi Tony,

You have asked two very good questions. Both of these are likely to spark some discussion and maybe even some controversy, so I'll answer them in two separate posts. I'll begin here by dicussing my preference for power MOSFETs over bipolar output transistors.

I won't be able to go into all the details here, but there are two places on my web site at www.cordellaudio.com where most of the answers to this question lie. First, look under Power Amplifier Design/Why I Prefer Power MOSFETs. Secondly, look in Section 1 of my paper, "A MOSFET Power Amplifier with Error Correction". Although this paper is perhaps best known for its application of error correction to MOSFET output stages, a very big part of the paper is about the application and advantages of power MOSFETs to audio amplifier output stages.

As in most engineering decisions, there is a tradeoff between choosing MOSFETs versus bipolars for an output stage. Each technology has its advantages and disadvantages. You pick your poison. It is possible to make an outstanding amplifier with either technology, and it is also possible to make a terrible amplifier with either technology. The best amplifiers are made by those who know best how to deal with the limitations of the devices they choose.

Take a bipolar transistor and give it infinite beta at all currents and give it a Vbe of about 3.5V instead of the usual 0.7V. Give it about one-tenth the transconductance of a normal bipolar transistor at a given current. Make it 5 times as fast as a fast ring-emitter (sometimes called a perforated emitter) transistor. Make it have no secondary breakdown, with a failure mechanism that is purely thermal in nature. Give it a peak current capability in excess of 50 amps. Make its current as a function of bias voltage about ten times less sensitive to temperature than a normal bipolar transistor. You now have a power MOSFET. Admittedly, this is a crude approximation, but it helps draw the lines of distinction between the two devices.

I like MOSFETs because they are fast and immune to secondary breakdown. Their equivalent ft is on the order of 300 MHz. As a result, they produce very little dynamic crossover distortion. Protection circuits can be simple. They can source incredible current, and don't need a driver capable of sourcing high current. They have far superior thermal bias stability to bipolars, even though at normal bias current levels they also have a modest but positive temperature coefficient of current as a function of gate voltage.

They do have disadvantages. The most notable is their lower transconductance at a given current as compared to bipolars. This leads to higher output impedance in a source-follower arrangement, which can lead to higher static crossover distortion. This is one reason that they benefit so much from the application of error correction.

Power MOSFETs still make very good output stages even without error correction, but they need to be biased somewhat hotter than a bipolar stage to keep the static crossover distortion small. They work quite well with a standing bias current of, say, 200 mA, which will result in a source-follower output impedance on the order of one ohm per device, or 0.5 ohm for the complementary pair. Note that this results in an output stage idle power dissipation of about 24 watts with +/- 60V rails. Of course, if you're building a big honking power amplifier, there is nothing stopping you from running three or four pairs, each at 200 mA, to greatly reduce static crossover distortion while having a very nice Class AAB design.

People typically parallel numerous pairs of bipolar output transistors. This is for two reasons. First, it is to mitigate the beta droop problem at high currents. Secondly, it is to build up a larger total output stage Safe Operating Area so that less-intrusive protection circuits can be used. There is less need for this with power MOSFETs.

Keep in mind that you should match power MOSFETs if you are going to parallel them. This is pretty easy, however, since today's power MOSFETs from the same tube are usually like peas in a pod.

Cheers,
Bob
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Old 14th November 2006, 12:26 PM   #3
PMA is offline PMA  Europe
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Bob,

are not you oversimplifying?

E.g., "do not need a driver capable of sourcing high current."

Really? In case input MOSFET capacitance is of some 1500pF, the driver has to deliver large

Ic = C*(dV/dt)

current, at least for fast signals.

Cheers,
Pavel
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Old 14th November 2006, 12:49 PM   #4
mlloyd1 is offline mlloyd1  United States
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pavel:

i agree with you. however, isn't some of this capacitance effectively "mitigated" when the MOSFET is used in a source follower configuration?

mlloyd1

Quote:
Originally posted by PMA
Bob,

are not you oversimplifying?

E.g., "do not need a driver capable of sourcing high current."

Really? In case input MOSFET capacitance is of some 1500pF, the driver has to deliver large

Ic = C*(dV/dt)

current, at least for fast signals.

Cheers,
Pavel
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Old 14th November 2006, 02:29 PM   #5
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Bob,

I guess your thoughts aplies mostly to vertical FET's, whats your take on lateral ones, not just paramters where the main differences like even lower transconductance, lower Cre and negative temp-co are the most noticeable but as well in practice?

Cheers Michael
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Old 14th November 2006, 04:13 PM   #6
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Bob;
I used FETs in parallel with BJTs, with emitter resistors selected such a way they work up to currents when their beta starts to roll off, the results were amazing.
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Old 14th November 2006, 04:50 PM   #7
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Default required current drive for MOSFETs

Quote:
Originally posted by PMA
Bob,

are not you oversimplifying?

E.g., "do not need a driver capable of sourcing high current."

Really? In case input MOSFET capacitance is of some 1500pF, the driver has to deliver large

Ic = C*(dV/dt)

current, at least for fast signals.

Cheers,
Pavel

Pavel, this is a good question, but I was not really oversimplifying. This issue was covered in Section 1.5 of my MOSFET amplifier paper. The gate-source capacitance is bootstrapped by the output, so if the source follower gain is, say, 0.9, the 700 pF or so of Cgs is effectively reduced to about 70 pF. Then remains the gate-drain capacitance of about 100 pf. Together they amount to an effective capacitance of about 170 pF, thus requiring about 17 mA to support a voltage slew rate of 100 V/us.

Looking at the input capacitance of a device by itself can be very misleading. A lot of people express concern about the input capacitance of MOSFETs when they don't realize how big the effective input capacitance of a bipolar transistor is. Consider a ring emitter transistor with an ft of 30 MHz and operating at 1 amp. What do you think the effective input capacitance is? It is a whopping 0.2 uF! Just remember that the hybrid pi input capacitance of a bipolar transistor is transconductance in Siemens divided by 2*pi*ft. The transconductance at 1 amp is 40 S. Even though this may seem not to be a "physical" capacitance, it matters every bit as much. The thing that mitigates the effect of this capacitance is the same thing that mitigates it for a MOSFET: in an emitter follower configuration the hybrid pi capacitance is bootstrapped to a much smaller effective value.

What I really meant about high current drive required for bipolar output stages was that required under high output current conditions when there is beta droop in the output stage. In some cases, you can have an output stage delivering 10 amps to the load under conditions where the beta has drooped to 20 or less. In this case the driver needs to supply over 500 mA to the output devices. THIS is what I meant by high drive current. I've seen cases where the bipolar drivers go into secondary breakdown before the output transistors.

Cheers,
Bob
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Old 14th November 2006, 05:00 PM   #8
PMA is offline PMA  Europe
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Bob,

thank you for the bright explanation. Regarding hFE vs. Ic decrease of BJTs, what do you think about 2SA1943/2SC5200 or their precedors 2SA1302/2SC3281, their beta is pretty linear up to some Ic = 8A.

Cheers,
Pavel
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Old 14th November 2006, 05:43 PM   #9
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Quote:
Originally posted by PMA
what do you think about 2SA1943/2SC5200 or their precedors 2SA1302/2SC3281, their beta is pretty linear up to some Ic = 8A.

What???

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Old 14th November 2006, 05:58 PM   #10
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What ? Pavel mean, that " old " ones ( 2N3055 for example )had much worse curve )....
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