Amplifier transistor matching?

Well, it might be useful to explain just why beta/hfe matching is important, considering that that the bipolar transistor is a voltage controlled device!

The issue is the internal base resistance, rbb', from the external base terminal to the actual junction.

Lets say, the output in a device is 5A, with a hfe of 100. This is 50ma base current. Typically, rbb' might be 5 ohms for a power device (or less). This results in 250 mv across rbb', that is, the applied voltage is reduced by 250mv. If the hfe was half due to mismatch, there would be a net 250mv difference in applied base emitter voltage *iff* the current stayed the same. It don't, as the current will be reduced resulting in less drop. The calculation actually gets a bit messy.

Essentially, we have:

IB1.RB1 + Vt.ln(IC1/Io1) = IB2.RB2 + Vt.ln(IC2/Io2)

simplifying with RB1=RB2 and Io1=Io2 we get

Vt.ln(IC1/IC2) = (IC2/Hfe2 - IC1/Hfe1).R

or

IC1/IC2 = exp((IC2/Hfe2 - IC2/Hfe1)R/Vt)

Which is still a bit tricky to solve, hence the introduction of SuperSpice:-)

We can actually do something more with the above with a bit of rearranging:

IC1.exp(IC1.R/Hfe1.Vt) = IC2.exp(IC2.R/Hfe2.Vt)

Which the more astute readers will recognise can be expresed in terms of our friend the Lambert W function,

IC1 = Vt.hfe1/R . W( R/(Vt.hfe1) . IC2.exp(IC2.R/Hfe2.Vt) )

So given, IC2 we can calculate IC1.

Emitter resisters introduce negative feedback, but I think I will stick to SS for the sums...

It should be noted that 2:1 hfe variations, without emitter degeneration can typically be of the order of 10:1 in current ratios.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

The modern Shure units have tone code squelch. When the mike is "muted" there is no pop or noise. There will be noise possibly when its first turned on. You should be be looking at the ULX series in SHure.

Bob

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A bi polar transistor is a current controlled device actually. A small base current is used to control a larger collector current. A FET is a voltage controlled device.

Bob

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I think i still have a Harmon Kardon 330 in the attic that uses TO 220's for outputs..... WOnder why its still siting there??? ;)

I seem to remember some home type amps that would have output distortion problems when the 1/2 watt emitter resistors in the driver stages went open.

Bob

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You know, I don't remember any more; it was around 15 years ago. To the extent I can dredge up any memories, it was a 5W ceramic cinderblock, so presumably wirewound.

Can I interject and ask some advice?

My wife has lots of Girl Scout speaking presentations, but she's a walker... walks away from the podium and the microphone.

Did it again last week with me frantically waving, "Go back to the microphone."

These presentations are usually in not-very-well or anciently equipped locations... last week was in an old Catholic Church Parish Center.

What should I buy in the way of a wireless microphone, with facility to plug the receiver into almost any PA equipment I might encounter?

Thanks!

...Jim Thompson

Now why would you guess that? I recently spent more than $1K on stencil-cutting and sandblasting equipment for one of her projects.

Joerg, What would you recommend?

...Jim Thompson

Nobody should be bound to the podium. Solve the problem.

I've used and can recommend the Sennies as well....

jak

older

battery

you'll

of a

the

Try the new G2 series Sennheisers. They use AA batteries and have 9hr battery life w/alkalines. With the new high capacity NiMh AA's out there you should get close to that or even longer life. They are not digital, but they do sound great. Why do you need true digital?

Mike D.

I think the original post is misleading you a bit. The 2245 amp will run just fine with non-matched output transistors, without 'sever distortion problems'. However, to achieve the lowest possible distortion and stability, matched pairs are advisable. This is true for *any* symmetrical audio amplifier design.

True. Especially for low NFB, low gain designs. Still the better the complimentary paiss match, the better will the amplifier be.

Well, when one side of a pair works while the other doesn't, you do get signal out, but it's seriously (50% and up) distorted. Not just 'poor fidelity' but 'the amp is definitely bad' kind of sound.

I recently had a similar problem with an 2270 output block. I replaced the bad output device with a matched (measured out of about a dozen) transistor. The receiver sounds great, but the repaird channel now measures better than the originally good one.

-- Ron

But rather more to the point, a driver stage that isn't low impedance will also cause the same problem.

Graham

Hello Walter,

That's right, except that 9V are a lot more expensive per Watt hour than AA batteries. But the real concern I have with 9V is that even brand new ones fail a lot. Happened again this week at church. After just a few minutes a brand new battery went from 9V to zero. They are just too fickle.

So even if we don't find any digital system the next mikes must be AA cells, no more 9V. Ideally they should be compatible with the Sennheiser UHF diversity receivers since we have four of them already.

Regards, Joerg

Hello Bob,

Tone squelch is a great concept but even without it isn't such a big deal to design it "pop free". I had an FM radio that never popped. It looks at the noise content on the audio signal to determine squelch action. I wonder why the wireless mikes couldn't do that.

When I repaired radios I sometimes looked at the schematics to see why they popped. Usually the designers didn't fully study the DC levels. There cannot be any DC shift or cap charge/discharge when the squelch kicks in. Other times they ignored the charge injection of the FET path used for muting.

Regards, Joerg

My wife has been a Girl Scout leader for 40 years ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

He relies on Christ. If He brings you to it, He will see you through it. John

Hello Graham,

now

Yes, I saw his post. That is just one more argument for abandoning 9V batteries. I am not at all satisfied with their quality levels and that is also a reason why I try to encourage my clients to design for AA batteries.

At our church we are also ushers so testing for 10 minutes just isn't easy to do. Also, we had batteries fail at all kinds of time frames. Some would die within minutes, others would go for 30-40 minutes and then die, and so on. From the 9V NiMH half of them died within months, some within weeks. Not good at all.

Indeed, but I have never heard the brand Vapextech. Anyway, I just don't want to continue with 9V anymore for any new mikes. For the existing one we continue do need them though so I'll check this one out. So thank you for that hint, it could help us get more mileage out of a charge.

Regards, Joerg

Did you see Ron's post in aapls ?

He's seen the same thing - battery packs up after a few mins. He 'tests' them now for 10-15 mins before use.

I was on ebay earlier and found some *280* mAh 9V NiMH cells. That's a little better than your current ones.

Seller seems to know what he's talking about too !

Graham