Old Uneducated Me Again, Transistor Circuit

I have designed and built a few things that worked. But I always used the sledge hammer approach when it comes to impedance. I made the output impedance of the stage much lower than the input impedance of the next. Upon further reflection, I think that sometime this is a waste and actually may be not all that effective.

These are the the throes of an almost old Man so don't tear me up.

I have known for a long time that the input impedance of a bipolar common collector stage is Re times hfe. But what about the common emitter stage ?

I have surmised that it is the same. Though lower, it is because of the selection of a lower value resistor to keep the voltage gain up. But as long as it operates in its linear region and that collector current is much more affected by base current rather than collector voltage, it is still Re times hfe.

The Re remains constant through all ranges, but the hfe does not. Since the base is current operated that means that feeding it with a solid (low impedance) voltage source can introduce non-linearity in the output because of variations of hfe within the operating range.

Tell me if I am wrong with my assertions. I am trying to better understand linear amplification in signals like audio and video, and instrumentation.

Am I totally wrong, partly wrong or actually right ?

 

[John Larkin]

On Mon, 5 Mar 2018 00:04:29 -0800 (PST), jurb6006@gmail.com wrote:

I have designed and built a few things that worked. But I always used the sledge hammer approach when it comes to impedance. I made the output impedance of the stage much lower than the input impedance of the next. Upon further reflection, I think that sometime this is a waste and actually may be not all that effective.

These are the the throes of an almost old Man so don't tear me up.

I have known for a long time that the input impedance of a bipolar common collector stage is Re times hfe. But what about the common emitter stage ?

The b-e junction of a transistor acts like a diode. Its small-signal
resistance is 25/Ib where Ib is the diode (base) current in milliamps.
That's the input impedance if the emitter is grounded or bypassed to
ground at signal frequencies. If there is an emitter resistor, add
beta*Re in series there.

More accurately, especially at high currents, you have to add the base
spreading resistance in the input side and the emitter contact
resistance on the emitter side. And there is also some feedback from
the collector circuit.

In most cases, it's usually easier to run LT Spice to do all that math
for you, although it is good to understand the principles.

Since individual transistors vary so much, calculating impedances
accurately isn't usually worth the trouble.



The book The Art of Electronics is worth reading. It explains all this
stuff in plain language. Old issues are cheap, or you can find it
online.

I have surmised that it is the same. Though lower, it is because of the selection of a lower value resistor to keep the voltage gain up. But as long as it operates in its linear region and that collector current is much more affected by base current rather than collector voltage, it is still Re times hfe.

The Re remains constant through all ranges, but the hfe does not. Since the base is current operated that means that feeding it with a solid (low impedance) voltage source can introduce non-linearity in the output because of variations of hfe within the operating range.

Tell me if I am wrong with my assertions. I am trying to better understand linear amplification in signals like audio and video, and instrumentation.

Am I totally wrong, partly wrong or actually right ?

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Phil Hobbs]

On 03/05/2018 03:04 AM, jurb6006@gmail.com wrote:

I have designed and built a few things that worked. But I always used
the sledge hammer approach when it comes to impedance. I made the
output impedance of the stage much lower than the input impedance of
the next. Upon further reflection, I think that sometime this is a
waste and actually may be not all that effective.

These are the the throes of an almost old Man so don't tear me up.

I have known for a long time that the input impedance of a bipolar
common collector stage is Re times hfe. But what about the common
emitter stage ?

I have surmised that it is the same. Though lower, it is because of
the selection of a lower value resistor to keep the voltage gain up.
But as long as it operates in its linear region and that collector
current is much more affected by base current rather than collector
voltage, it is still Re times hfe.

The Re remains constant through all ranges, but the hfe does not.
Since the base is current operated that means that feeding it with a
solid (low impedance) voltage source can introduce non-linearity in
the output because of variations of hfe within the operating range.

Tell me if I am wrong with my assertions. I am trying to better
understand linear amplification in signals like audio and video, and
instrumentation.

Am I totally wrong, partly wrong or actually right ?

You're right as concerns low frequencies.

At high frequency, the input impedance goes down on account of the
Miller effect.

There's some capacitance C_cb between base and collector. In a
common-collector (i.e. emitter follower) circuit, only the base end
movs, so C_cb appears between base and ground.

In a common-emitter stage with gain -A, the base moves the same way but
the collector moves A times further in the other direction, so that the
voltage across C_cb is A+1 times bigger. From the base's point of view,
that looks exactly as though C_cb has been multiplied by A+1. That's
the Miller effect.

The usual ways of getting rid of it are to drive the CE stage from a
follower or adding a common base stage to the output. The first
supplies a low drive impedance to overcome the extra capacitance, and
the second holds the collector nearly still, so that the voltage gain
seen from the base is very low (no more than 2, and usually much less).

In the cascode, the Miller effect still operates on the upper
transistor, but since its base is bypassed to ground, it doesn't slow
the stage down.

There's also the Early effect, which reduces the gain of the stage a bit
(typically 5-20% depending on the device) but is usually less of a
problem than Miller.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com

 

[Helmut Wabnig]

On Mon, 5 Mar 2018 00:04:29 -0800 (PST), jurb6006@gmail.com wrote:

I have designed and built a few things that worked. But I always used the sledge hammer approach when it comes to impedance. I made the output impedance of the stage much lower than the input impedance of the next. Upon further reflection, I think that sometime this is a waste and actually may be not all that effective.

These are the the throes of an almost old Man so don't tear me up.

I have known for a long time that the input impedance of a bipolar common collector stage is Re times hfe. But what about the common emitter stage ?

I have surmised that it is the same. Though lower, it is because of the selection of a lower value resistor to keep the voltage gain up. But as long as it operates in its linear region and that collector current is much more affected by base current rather than collector voltage, it is still Re times hfe.

The Re remains constant through all ranges, but the hfe does not. Since the base is current operated that means that feeding it with a solid (low impedance) voltage source can introduce non-linearity in the output because of variations of hfe within the operating range.

Tell me if I am wrong with my assertions. I am trying to better understand linear amplification in signals like audio and video, and instrumentation.

Am I totally wrong, partly wrong or actually right ?

Don't know if I understand your question,
but with audio it's always a voltage follower.
Low output impedance and high load impedance.
Therefore a small change in load impedance does not matter at all.

w.

 

[Phil Allison]

Helmut Wabnig wrote:

-------------------

jurb6006@gmail.com wrote:

I have designed and built a few things that worked.

** No kidding ...........

But I always used the sledge hammer approach when it comes to impedance.
I made the output impedance of the stage much lower than the input
impedance of the next. Upon further reflection, I think that sometime
this is a waste and actually may be not all that effective.

** Actually it is at the heart of electronics to do that.

Vacuum tubes, from the first triodes exhibited input impedances in the
order of Gohms and output impedances in the thousands of ohms. This made them useful amplifiers with enormous power gains in a single stage.

Tell me if I am wrong with my assertions. I am trying to better
understand linear amplification in signals like audio and video,
and instrumentation.

Am I totally wrong, partly wrong or actually right ?


Don't know if I understand your question,
but with audio it's always a voltage follower.

** A "voltage follower" is a buffer stage with unity voltage gain - like an emitter follower or cathode follower.

Power gains may however be enormous. The JFET or miniature triode tube at the head of a condenser mic is a voltage follower / impedance converter with a million to 1 power increase.

Low output impedance and high load impedance.
Therefore a small change in load impedance does not matter at all.

** An idea used by lots of test equipment like scopes, VTVMs and modern DMMs.



...... Phil





**

 

>"Don't know if I understand your question,
but with audio it's always a voltage follower.
Low output impedance and high load impedance.
Therefore a small change in load impedance does not matter at all. "

Your statement seems limited to the output stages. What about the voltage gain stages ?

 

"> Low output impedance and high load impedance.
Therefore a small change in load impedance does not matter at all.

** An idea used by lots of test equipment like scopes, VTVMs and modern DMMs. "

Hold on, this goes against one of my assumptions. (I THINK) That appears it will work fine if the next stage is an FET, or a bipolar with a very smooth hfe/Ic characteristic. However in practice the hfe can vary considerably though the operating range. As the input signal passes through this range the hfe variations would cause changes in Ib, correct ? As such that would introduce nonlinearity in the output of the following stage, no ?

That nonlinearity is obviously undesirable in instrumentation, or in the instance when one wants to design audio equipment that uses less feedback. Such low feedback designs are touted as sounding better. I don't have proof but it seems that negating a huge amount of open loop gain might cause some sort of difficult to measure distortion or whatever.

Increasing speaker damping can be easily handled by a lower impedance source feeding the outputs and drivers, but a voltage gain stage is a bit different and that is what the feedback affects.

Maybe I put it wrong as I sometimes do. If so say so.

 

[Phil Allison]

jurb...@gmail.com wrote:

-------------------------

"> Low output impedance and high load impedance.
Therefore a small change in load impedance does not matter at all.


** An idea used by lots of test equipment like scopes, VTVMs and modern DMMs. "

Hold on, this goes against one of my assumptions.

** Only one ? Surely you jest.

The idea is to build gain stages that can be connected one after the other with predictable results. When each has a low output impedance compared to its input impedance, then this is the result.

Making each stage LINEAR and with WIDE bandwidth is a separate issue. Until op-amps took over, gain stages were made from 2 or 3 transistors or tubes.

Maybe I put it wrong as I sometimes do. If so say so.

** You need to put an effort into grasping the basics and not expect folk to decipher your tortured mental meanderings.



..... Phil