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Amplifier transistor matching?

K

Kevin Aylward

Jan 1, 1970
0
Pooh said:
These devices which I currently use for example are pre-graded by the
manufacturer. Worst case match is 2:1 in either gain grade.

http://www.profusionplc.com/cgi-bin/gex/pcatdtl?ipartno=2SC5200-O

A simple low value emitter ballast resistor overcomes the bulk of beta
mismatch anyway and I would never fail to use them. You can't depend
on paralled device temps being identical - in fact quite the reverse
- never mind thermal runaway !

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,
http://www.anasoft.co.uk/EE/widlarlambert/widlarlambert.html, to wit:

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
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
T

Trevor Wilson

Jan 1, 1970
0
Midlant said:
Cor, I too have one of these. Is yours a made in California real Marantz
or a later one made in Japan. I have a scratchy right channel. I've
cleaned the pots, especially the volume pot, but it doesn't seem to have
helped. Have you had this problem? If so what did you do to rectify it?
John

**No Marantz 2245 was made in the US. ALL 4 digit models were of Japanese
origin. Check for low level DC Voltage on the pot. If present, you have a
coupling cap fault. If not, you have a 'dirty' pot. Clean it.
 
C

cor

Jan 1, 1970
0
Midlant said:
Cor, I too have one of these. Is yours a made in California real Marantz
or a later one made in Japan. I have a scratchy right channel. I've
cleaned the pots, especially the volume pot, but it doesn't seem to have
helped. Have you had this problem? If so what did you do to rectify it?
John

Mine is made in Japan. Ser 27960.
Maybe that it why the manual I got does not exactly match the components I have.

My pots seem to be ok. I only noticed that the on button is slow in the way out.

Are you sure its the pots? If you switch sides at the preamp plugins does the
scratchiness switch sides?
 
B

Bob Urz

Jan 1, 1970
0
Joerg said:
Hello Graham,



The analog ones are ok, except for a suboptimal squelch and a nasty pop
when muting it. In church you have to do that a lot.
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
 
B

Bob Urz

Jan 1, 1970
0
Kevin said:
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!
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
 
B

Bob Urz

Jan 1, 1970
0
Pooh said:
Walter Harley wrote:




TO-220s ! Those are driver transistors ! ;-)
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
 
P

Pooh Bear

Jan 1, 1970
0
Kevin said:
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.

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

Graham
 
J

Joerg

Jan 1, 1970
0
Hello Walter,
Dunno about digital. The Shure analog UHF packs, with lav mics, last more
than 6 hours with a pair of alkaline AAs. One of my gigs uses a dozen or
more channels of them; we put fresh batteries in at 4:30pm, and at 10:30pm
when the show ends they're usually still showing three or four out of five
bars on the battery life indicator. We replace them every night anyway - if
we went for two nights, by the end of the second night we'd be too nervous.
As jak said, the price of batteries is small compared to the price of the
show going down.

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
 
W

Walter Harley

Jan 1, 1970
0
It was a film resistor that failed rather than wire wound I assume ?

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.
 
J

Joerg

Jan 1, 1970
0
Hello Bob,
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.

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
 
J

Jim Thompson

Jan 1, 1970
0
Dunno about digital. The Shure analog UHF packs, with lav mics, last more
than 6 hours with a pair of alkaline AAs. One of my gigs uses a dozen or
more channels of them; we put fresh batteries in at 4:30pm, and at 10:30pm
when the show ends they're usually still showing three or four out of five
bars on the battery life indicator. We replace them every night anyway - if
we went for two nights, by the end of the second night we'd be too nervous.
As jak said, the price of batteries is small compared to the price of the
show going down.

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
 
P

Pooh Bear

Jan 1, 1970
0
Joerg said:
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.

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.
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.

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

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=50622&item=5779432419&rd=1

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

Graham
 
P

Pooh Bear

Jan 1, 1970
0
Walter said:
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.

Oh - unusual but wth !

I ended up using flameproof power film resistors for emitter Rs so that in
the event of catastrophic failure - a cascade failure or 'burn up' as our
repair guys used to call it, the emitter Rs went open fast. Usually little
damage in that event. I do recall seing some TO-92s with their 'heads'
popped off though !

Graham
 
P

Pooh Bear

Jan 1, 1970
0
Jim said:
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?

About time to start a new thread ?

It all depends. Depends on your budget and the quality you're looking for.

I'm guessing that budget is low in your case. You *can* get cheap 'voice quality'
radio mics but these aren't a patch on the Sennheisers that Joerg is using.

You get what you pay for for the most part. The receiver should have no trouble
interfacing with any kind of PA gear btw.

I'm tempted to suggest looking on ebay for a cheap unit.

Graham
 
J

Jim Thompson

Jan 1, 1970
0
About time to start a new thread ?

It all depends. Depends on your budget and the quality you're looking for.

I'm guessing that budget is low in your case.

Now why would you guess that? I recently spent more than $1K on
stencil-cutting and sandblasting equipment for one of her projects.
You *can* get cheap 'voice quality'
radio mics but these aren't a patch on the Sennheisers that Joerg is using.

You get what you pay for for the most part. The receiver should have no trouble
interfacing with any kind of PA gear btw.

I'm tempted to suggest looking on ebay for a cheap unit.

Graham

Joerg, What would you recommend?

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
Hello Graham,
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.

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

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=50622&item=5779432419&rd=1

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

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
 
J

Joerg

Jan 1, 1970
0
Hello Jim,
Joerg, What would you recommend?

Assuming that you don't want to spend a whole lot and that it doesn't
have to be hifi I would look at a Radio Shack setup. That is what we had
when our church was just starting (aka lower in budget...). We also used
an RS wireless mike for large meetings with production employees at my
last company. We had to hold those in the cantina since it was the only
place where the fire marshall allowed enough occupancy.

The units are different now but still cheap:

http://www.radioshack.com/category....1&find=wireless microphone(keyword)&hp=search

One of them is a complete set, mike and receiver. Then you only have to
plug the receiver's line out into the PA system. The older one we had
could also be operated from batteries and from a car battery which would
be really nice for scout meetings in the outbacks.

Regards, Joerg
 
M

Mike Dobony

Jan 1, 1970
0
Joerg said:
Hello Graham,


The analog ones are ok, except for a suboptimal squelch and a nasty pop
when muting it. In church you have to do that a lot.


I had asked in rec.audio.pro but I'll try your suggestion tomorrow. Not
today, the barbie is almost ready. Marinated ribs and potatoes tonight.

We use Ansmann 9V NiMH which seem to be the only ones with 250mAh, plus
nifty uC charge stations. But even with top notch Alkalines our
Sennheiser EW system doesn't reach 5hrs. Actually the Ansmanns hold out
a bit better. Thing is, two AA cells pack a whole lot more energy than a
9V battery. I wonder why they didn't design for 3V or even better 2.4V.

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.
 
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