Re: Will this transistor amplifier work in production?

From: Jim Thompson (thegreatone_at_example.com)
Date: 06/30/04 

Date: Wed, 30 Jun 2004 08:39:34 -0700

On Wed, 30 Jun 2004 11:07:41 -0400, "Tam/WB2TT"
<t-tammaru@c0mca$t.net> wrote:

>
>"The other John Smith" <jocjo-john@yooha.com> wrote in message
>news:_3oEc.18226$w07.3777@newsread2.news.pas.earthlink.net...
>>
>> "The other John Smith" <jocjo-john@yooha.com> wrote in message
>> news:z5mEc.5403$lh4.4654@newsread1.news.pas.earthlink.net...
>> > This circuit works very well on my breadboard...
>> >
>> > +5V
>> > .-------------------o
>> > | |
>> > .-. .-.
>> > | | | |
>> > | | 9.1k | | 4.7k
>> > '-' '-'
>> > | |
>> > | |
>> > | o----- OUT
>> > o---. |
>> > | | |
>> > | | |
>> > \| | |/
>> > 2N3904 |-o---Source----| 2N3904
>> > <| |>
>> > | |
>> > | |
>> > |-------------------'
>> > |
>> > ===
>> > GND
>> > created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
>> >
>> > The source is about 0.6 Hz at 15 mV p-p and has about 500 Ohms internal
>> > resistance. The circuit provides a voltage gain of about 80 and feeds a
>> > comparator whose reference is about 1/2 the supply voltage. I ran it
>> through
>> > LT Spice and it seems to work just fine over -40 to 60 C. The resistor
>> > values shown are values required by LT Spice to set the output at about
>> half
>> > the supply voltage with the signal at 0 volts. When I implemented the
>> > breadboard, the 9.1k needed to be reduced to about 6k. This got me to
>> > wondering if I will have to select resistors during production to keep
>the
>> > output voltage at about 1/2 the supply voltage. I know I could use an
>> op-amp
>> > for this, but now I'm wondering if this is a practical circuit.
>> >
>> > Any comments about this circuit will be greatly appreciated.
>>
>>
>>
>> Wow! Thanks for all the great replies!
>>
>> It looks like I might have a chance with this circuit if I use matched
>> pairs. And the component count is the same whether I use an op-amp,
>> comparator, or matched transistors in a single package.
>>
>> Probably best to stick with the usual op-amp or comparator, though.
>>
>> Thanks again for the help.
>>
>> John
>>
>>
>John,
>
>How are you coupling the input signal? I ran LTSPICE on this, capacitively
>coupling the signal into the bases through a 10 uf capacitor, and saw about
>what you say. Now, put a resistor in series with the input (I used 9.1K),
>and the picture changes completely. I think the impedance looking into the
>base of the left hand transistor is very low because of the unity inverse
>feedback. I wasn't very careful, but the impedance into the amp appears to
>be less than 30 Ohms. I think what I am seeing is the re of the left hand
>transistor.
>
>Tam
>

John said (and illustrated above) the source as **between** the bases.

Use something like a CA3046 and it'll work just ducky.

With a CA3046 it should be possible to make the output very accurately
at Vcc/2 and stable over temperature.

If I have time this morning I'll doodle up something and post it to
either my website or to a.b.s.e

                                        ...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  |
|       http://www.analog-innovations.com           |    1962     |
             
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