Voltage follower with a step function (need a design for one)

[Fred Bloggs]

Rocky wrote:

Thank you everyone for you contributions to my question. In summary
I'll probably use either John Fields' or Tony's circuit because they
are single chip solutions. They are both similar in their approach,
just that John used a comparator in place of Tony's transistor to
pull down the input when 0<Vin<2V. Jim Thompson's suggestion was kind
of similar, although it uses a 4066 analog switch to gate the signal.
I haven't yet wrapped my head around how Fred Blogg's circuit works,
but given the higher active component count I probably wouldn't use
it unless it has specific advantages that I'm not aware of.

The circuit is a true function circuit without hysteresis, something
impossible to attain with a comparator. The circuit is redrawn below for
explanatory purposes. The block labeled "feedback" is a well-known
configuration that outputs the larger of the two IN(+) input voltages on
U2 and U3, or Vf=MAX(Vout,2V) in the notation shown. Among other things,
this means that Vf= IN(-) node of U1 is always >=2V, so that for Vin<2V,
U1 is driven into saturation at its most negative rail- 0V in the case
of single supply. For Vin>2V, U1 will drive its output towards the
positive rail in the amount necessary to force its IN(+)-IN(-)=0, or
Vf=Vin. This means the circuit produces Vin=Vf=MAX(Vout,2V)=Vout for
Vin>2V. The circuit will amplify the ever present noise for Vin
exactly=2V , and if this is a problem then a small amount of resistive
feedback from Vout to U1 IN(+) will remove the output noise response.

View in a fixed-width font such as Courier.

 

[John Fields]

I downloaded LTSpice and opened up the above text as a circuit file
(*.cir), when I ran it, I got the error message
"Circuit: Version 4

Fatal Error: Multiple instances of "Symattr"

Do you know why?

 

[Rocky]

The circuit is a true function circuit without hysteresis, something
impossible to attain with a comparator.... <snip>....
View in a fixed-width font such as Courier.

.
.
. feedback
. -------------------------
. | |
. | .---------------. |
. | | | |
. | | U3/| | |
. | | /-|--' |
. | +---|<|---< | |
. | | \+|---<2V |
. | | \| |
. | | |
. | +---------------. |
. | | | |
. | | U2/| | |
. | | /-|--' |
. | +---|<|---< | |
. | | \+|--. |
. | | \| | |
. -------------------------
. | |
. |Vf |
. | U1 |
. | |\ |
. Vin>--------[10K]---|+\ |
. | | >--+--------> Vout
. +-[10K]-+-|-/ |
. | | |/ |
. [2.2K] | 3.3N |
. | '--||---'
. ---
. ///
.
.

Do not snip this ASCII art in any of your replies. If you do- I will
repost it x10 back in your face. If you can't respond politely or
knowledgeably then don't respond at all.

Thanks for the reply.
So is your "do not snip" directed at everyone or someone specific? I
have no idea what prompted this threat of yours.

Rocky

 

[Fred Bloggs]

The circuit is a true function circuit without hysteresis, something
impossible to attain with a comparator.... <snip>....
View in a fixed-width font such as Courier.

.
.
. feedback
. -------------------------
. | |
. | .---------------. |
. | | | |
. | | U3/| | |
. | | /-|--' |
. | +---|<|---< | |
. | | \+|---<2V |
. | | \| |
. | | |
. | +---------------. |
. | | | |
. | | U2/| | |
. | | /-|--' |
. | +---|<|---< | |
. | | \+|--. |
. | | \| | |
. -------------------------
. | |
. |Vf |
. | U1 |
. | |\ |
. Vin>--------[10K]---|+\ |
. | | >--+--------> Vout
. +-[10K]-+-|-/ |
. | | |/ |
. [2.2K] | 3.3N |
. | '--||---'
. ---
. ///
.
.

Do not snip this ASCII art in any of your replies. If you do- I will
repost it x10 back in your face. If you can't respond politely or
knowledgeably then don't respond at all.

Thanks for the reply.
So is your "do not snip" directed at everyone or someone specific? I
have no idea what prompted this threat of yours.

Rocky

I don't know anything about you in particular, but SED is loaded with
parasitic leeches who love to insult or otherwise provoke people
indirectly by doing that kind of thing, and at the same time, these
parasitic leeches will maintain huge quotes in off topic threads
comprised of pure babble.

 

[Winfield Hill]

Fred Bloggs wrote...

Do not snip this ASCII art in any of your replies. If you do-

Calm down Fred.

 

[ehsjr]

Fred Bloggs wrote...



Calm down Fred.

Yabut ... Fred posts great stuff. Wouldn't it be
nice if, at least once in a while, the op's answer
was: "Wow! Thanks a lot ..." or "Geez - your circuit
works great..." or "Thanks for the help! Could you
explain ...?" or something like that?

Ed

 

[Rocky]

The circuit is a true function circuit without hysteresis, something
impossible to attain with a comparator.... <snip>....
View in a fixed-width font such as Courier.

.
.
. feedback
. -------------------------
. | |
. | .---------------. |
. | | | |
. | | U3/| | |
. | | /-|--' |
. | +---|<|---< | |
. | | \+|---<2V |
. | | \| |
. | | |
. | +---------------. |
. | | | |
. | | U2/| | |
. | | /-|--' |
. | +---|<|---< | |
. | | \+|--. |
. | | \| | |
. -------------------------
. | |
. |Vf |
. | U1 |
. | |\ |
. Vin>--------[10K]---|+\ |
. | | >--+--------> Vout
. +-[10K]-+-|-/ |
. | | |/ |
. [2.2K] | 3.3N |
. | '--||---'
. ---
. ///
.
.

Do not snip this ASCII art in any of your replies. If you do- I will
repost it x10 back in your face. If you can't respond politely or
knowledgeably then don't respond at all.

Thanks for the reply.
So is your "do not snip" directed at everyone or someone specific? I
have no idea what prompted this threat of yours.

Rocky

I don't know anything about you in particular, but SED is loaded with
parasitic leeches who love to insult or otherwise provoke people
indirectly by doing that kind of thing, and at the same time, these
parasitic leeches will maintain huge quotes in off topic threads
comprised of pure babble.

What on earth are you talking about Fred? I am at a loss so see how I

1. responded impolitely to you, or
2. insulted you, or
3. provoked you.

if you are going to make such threats and liken my actions to those
done by a parasitic leech, the least you could do is make yourself
clear about what exactly the problem is. Please refer to the EXACT post
of mine which offended you and tell me WHY. Only then can I see how I
may have done something wrong in your eyes.

Rocky

 

[Rocky]

Huh? As the original poster , my first response to Fred was to ask him
how the circuit worked because I didn't immediately see it. There's no
way I'm about to sing the praises of something I don't understand. So
then Fred goes on to explain his circuit, and in the same breath
insults me for goodness knows what.... So I reply by saying thanks
(for the explanation) and also asking how I had offended him. Is that
still a problem for you Ed?

Rocky

 

[Rocky]

I downloaded LTSpice and opened up the above text as a circuit file

Yes. ".cir" is LTSPICE's filename extension for a netlist. Rename it
with a ".asc" extension and it should run.

Thanks John, this was indeed the problem, I changed the extension to
..asc and it ran fine.
So now I have had a chance to simulate both the circuits, and I too can
see what you mean. The Vce_sat is definitely lower when the collector
is grounded rather than the emitter. I got identical voltages to those
listed by Jonathan Kirwan above. I'm not used to working with
transistors designed as switches, I've really only used them as signal
amplifiers.
Even though this thread is now getting rather off the OT (because such
a small difference in saturation voltage will not confer a noticeable
advantage to the main circuit ) , I still don't understand what Ken
Smith meant when he said "If the max signal voltage is under about 7V,
swap Q1's E and C legs." I tested the circuit with Vin=12V and the
cct. with the collector grounded still had a lower Vce_sat. pm me if
you want a copy of either cct or screen shot of the sim.

regards
Rocky

 

[ehsjr]

Huh? As the original poster , my first response to Fred was to ask him
how the circuit worked because I didn't immediately see it. There's no
way I'm about to sing the praises of something I don't understand. So
then Fred goes on to explain his circuit, and in the same breath
insults me for goodness knows what.... So I reply by saying thanks
(for the explanation) and also asking how I had offended him. Is that
still a problem for you Ed?

Rocky

"Is that still a problem for you Ed?"

Hi Rocky,

The above would set some people off. Me - I don't
care. But just so you know, I don't have a problem,
and did not have a problem. And I have no interest
in who insulted/did not insult or whatever the heck
is or is not going on with regards to insults.

My post to Win actually had nothing to do with what
you posted. I just shake my head in wonder at the
ratio of *excellent* answers from Fred to the number
of replies that express appreciation. Fred's
phrase "parasitic leeches" - having nothing to do
with you - may be a perfect description of those
who post, get great replies, and never say a word.
I'll add that this happens to others besides Fred.

Ed

 

[Andrew M]

Hi there,
I would like to effect the following transfer function (seen below in
ASCII art) using analog components such as some opamp(s), and/or diodes
etc. I'm not too fussy about the input and output impedances since I
can buffer the design. Here is a brief description of the function:
As the input voltage increases from 0V to +4V , I would like the output
to track (i.e. follow) the input voltage only after the input voltage
has exceeded a user adjustable threshold (nominally about 2V). The
function should have no (or minimal) hysteresis -i.e. behave the same
whether Vin ramps from 0-4V or from 4-0V. It does not need a high
bandwidth, but should operate from DC to about 20Hz.


Vout
^
4V | *
| *
| *
| *
| *
2V | *
| *
| *
| *
| *
|*********
-------------------------> Vin
' ' '
0 2V 4V


regards
Rocky

I'm not sure if anyone has asked this - Why? What is your objective.

A more creative response can be gained from an objective question.

-Andrew M

 

[Rocky]

"Is that still a problem for you Ed?"

Hi Rocky,

The above would set some people off. Me - I don't care.

Thanks Ed. I am still racking my brains to figure out what Fred thinks
I actually did wrong in his eyes. It may be blinking obvious to you
and others reading this thread - but if you know (i.e. when you say
"The above would set some people off" what do you mean), could you
spell it out for me? Sorry if I'm being persistent or trivial here,
but Fred hasn't replied back to tell me what I did/said to annoy him
and I don't like leaving things undone.

My post to Win actually had nothing to do with what
you posted. I just shake my head in wonder at the
ratio of *excellent* answers from Fred to the number
of replies that express appreciation. Fred's
phrase "parasitic leeches" - having nothing to do
with you - may be a perfect description of those
who post, get great replies, and never say a word.
I'll add that this happens to others besides Fred.

Ed

OK, I understand where you were coming from now.

thanks
Rocky

 

[Andrew M]

Hi there,
I would like to effect the following transfer function (seen below in
ASCII art) using analog components such as some opamp(s), and/or diodes
etc. I'm not too fussy about the input and output impedances since I
can buffer the design. Here is a brief description of the function:
As the input voltage increases from 0V to +4V , I would like the output
to track (i.e. follow) the input voltage only after the input voltage
has exceeded a user adjustable threshold (nominally about 2V). The
function should have no (or minimal) hysteresis -i.e. behave the same
whether Vin ramps from 0-4V or from 4-0V. It does not need a high
bandwidth, but should operate from DC to about 20Hz.


Vout
^
4V | *
| *
| *
| *
| *
2V | *
| *
| *
| *
| *
|*********
-------------------------> Vin
' ' '
0 2V 4V


regards
Rocky

Well there are some sophisticated designs in reply to your post.

However I can suggest a simpler, cheaper, more flexible and reliable
one. I notice that you have specified analogue components only - why
is this? This would be an *excellent* starter project for
microcontrollers.

Find a small microcontroller with ADC and PWM (PIC10F222 would be OK
with software PWM). Just model your step function in a lookup table.
The bandwidth you have specified can easily be achieved using filtered
PWM.

1) sample the input
2) lookup output
3) send output to PWM or DAC
4) goto 1)

These things are FLASH single chip, internally clocked, SOT23-6,
3.3/5V and cost way less than a dollar. An Aussie dollar at that.

-Andrew M


By the way, what's this for? - it looks like an exam question.

 

[Rocky]

I'm not sure if anyone has asked this - Why? What is your objective.

A more creative response can be gained from an objective question.

-Andrew M

Actually no one yet has asked, and I was particular to word the
question in a way that provided just the necessary details for a
suitable (not necessarily perfect) solution. I am more than happy with
the solutions given and in fact last night purchased the components
necessary to implement it. But since you asked I'm more than happy to
expand on its use.
I know a couple who are having their first child. The mother to be is
profoundly deaf and can not benefit from a conventional (wireless)
audio baby monitor in order to arouse her from sleep to feed the child.
I am making a circuit which takes the audio signal from the
conventional baby monitor and converts it into a signal to drive a very
low frequency tactile/vibration transducer which she can place under
her pillow. The advantage of my design is that the amplitude of the LF
vibrations are proportional to the amplitude of the audio signal (hence
proportional to the volume of the baby's cry). This way she can better
sense whether the baby needs attention or not. Using this in
combination with a commercially available wireless video receiver will
also help her to determine the child's needs.
At a lower level, here is how the circuit works:

The audio signal from the baby monitor is adjusted to be approx 1V_peak
and is then passed through a precision peak detector (with about 250ms
decay time) to give a low frequency signal (labelled Vin in the graph
in my first post) proportional to the amplitude of the original audio
signal.
A separate oscillator (XR-2206 monolithic func. gen IC) is set up to
generate a sine wave to drive the tactile/vibration transducer at a
frequency adjustable between 5-30Hz. This sine wave is amplitude
modulated by Vin. The XR-2206 was chosen because it already had an AM
pin on it (pin 1) which responded to +/-4V about VDD/2 (where VDD is
the single positive supply rail). As Vin goes from (VDD/2)V to (VDD/2
+4)V, the sine wave's amplitude goes from 0 to maximum. Now here is
where I needed the circuit I asked for. Sometimes it is not very
useful to be woken up when the baby is just making noises in their
sleep, so I needed a noise gate to prevent murmurs from waking up the
mother. To achieve this I needed to only pass the value of Vin through
to the AM pin when Vin exceeded a certain threshold (in this case 2V,
but it can easily be a user adjustable sensitivity). So there you
have it, the application for my mystery circuit. I hope some of you
have found it an interesting read.

regards
Rocky

 

[Winfield Hill]

Rocky wrote...

"Is that still a problem for you Ed?"

Hi Rocky,

The above would set some people off. Me - I don't care.

Thanks Ed. [snip]

My post to Win actually had nothing to do with what
you posted. I just shake my head in wonder at the
ratio of *excellent* answers from Fred to the number
of replies that express appreciation. Fred's
phrase "parasitic leeches" - having nothing to do
with you - may be a perfect description of those
who post, get great replies, and never say a word.
I'll add that this happens to others besides Fred.

OK, I understand where you were coming from now.

I agree, but I still think Fred should calm down.
Life is too short to waste any of it being upset.

 

[Rocky]

Well there are some sophisticated designs in reply to your post.

However I can suggest a simpler, cheaper, more flexible and reliable
one. I notice that you have specified analogue components only - why
is this? This would be an *excellent* starter project for
microcontrollers.

I'm more comfortable with designs in analogue right now, programming a
PIC would need more of a learning curve for me. I knew the design was
easily done in analog, but the solution was evading me.

Find a small microcontroller with ADC and PWM (PIC10F222 would be OK
with software PWM). Just model your step function in a lookup table.
The bandwidth you have specified can easily be achieved using filtered
PWM.

1) sample the input
2) lookup output
3) send output to PWM or DAC
4) goto 1)

These things are FLASH single chip, internally clocked, SOT23-6,
3.3/5V and cost way less than a dollar. An Aussie dollar at that.

-Andrew M

Nice idea, but time is not on my side right now to learn to program
this PIC.

By the way, what's this for? - it looks like an exam question.

See my last post for a description of what its for. I guess the fact
that it looks like an exam question is a result of me wanting to
specify the requirements in a way that really nails them down. In an
exam you don't want multiple interpetations of the problem.

Rocky