MOSFET switch problem

S

Saran

Guest
Hey all,

I feel dumb to ask this but I am having a very very (what I think is)
basic MOSFET switch question. Can anyone tell me what the output Vout
of the following schematic is going to be..



Vdc = -3V

|
|
|
|
---+---
+----+
VIN | |
+-------+ +------+- VOUT
| |
| |
| |
| -+-- c = 10pF
-+---
vdc = 3V |
| | l
| |
Gnd
GND

I would expect the gate to be turned off and the Vout should be ideally
zero + some leakage...I am trying to design switches for my DRAM which
pass the voltage input to the output of the pass transistor when the
control signal to the gate is turned on. I am trying to store that
voltage on the capactitor. However, the problem I am getting is that
the MOSFET is always on irrespective of the control gate voltage and I
am always getting my Vout to follow Vin (almost) whatever I do...

I went back to my basics checked everything but I am really stumped.
Can anyone change my line of thinking ? What am I doing wrong here???

Thanks a lot,
Saran
 
Saran wrote:
Hey all,

I feel dumb to ask this but I am having a very very (what I think is)
basic MOSFET switch question. Can anyone tell me what the output Vout
of the following schematic is going to be..



Vdc = -3V

|
|
|
|
---+---
+----+
VIN | |
+-------+ +------+- VOUT
| |
| |
| |
| -+-- c = 10pF
-+---
vdc = 3V |
| | l
| |
Gnd
GND

I would expect the gate to be turned off and the Vout should be ideally
zero + some leakage...I am trying to design switches for my DRAM which
pass the voltage input to the output of the pass transistor when the
control signal to the gate is turned on. I am trying to store that
voltage on the capactitor. However, the problem I am getting is that
the MOSFET is always on irrespective of the control gate voltage and I
am always getting my Vout to follow Vin (almost) whatever I do...

I went back to my basics checked everything but I am really stumped.
Can anyone change my line of thinking ? What am I doing wrong here???

Thanks a lot,
Saran
Click to expand...
You are designing an analog switch. Take a look at "The Art of
Electronics", chapter 3, for some useful tips. Alternately, you can buy
analog switches in DIP packages. 4066 comes to mind.

http://www3.sympatico.ca/lloyd.gordon3/images/4066.pdf

Many mosfets have built-in diodes, which will cause problems in this
application if you aren't careful. In your drawing (assuming it's an
N-channel MOSFET) if VOUT is supposed to stay high, and VIN is low, the
cap will drain through the diode no matter what the gate voltage is. You
can defeat this by using back-to-back MOSFETs, or use JFETs, which don't
have the diode.

--
Regards,
Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
Thomas Henry Huxley, 1877
 
Thanks a lot Bob, for your suggestion. I understand I am trying to
design an analog switch. But, the fundamental concepts I know clashes
with the simulation results I am observing when I simulated the
_schematic_in_question_.

If I apply a gate voltage to turn the device off....wouldn't the
voltage on the capacitor be zero ideally? How will Vin be reflected at
Vout? I know I must be missing something here...but I am not able to
see what I am missing. I will go thru AOE one more time in regards to
ur suggestion, thanks.

Regards,
Saran
 
Saran wrote:

Hey all,

I feel dumb to ask this but I am having a very very (what I think is)
basic MOSFET switch question. Can anyone tell me what the output Vout
of the following schematic is going to be..



Vdc = -3V

|
|
|
|
---+---
+----+
VIN | |
+-------+ +------+- VOUT
| |
| |
| |
| -+-- c = 10pF
-+---
vdc = 3V |
| | l
| |
Gnd
GND

I would expect the gate to be turned off and the Vout should be ideally
zero + some leakage...I am trying to design switches for my DRAM which
pass the voltage input to the output of the pass transistor when the
control signal to the gate is turned on. I am trying to store that
voltage on the capactitor. However, the problem I am getting is that
the MOSFET is always on irrespective of the control gate voltage and I
am always getting my Vout to follow Vin (almost) whatever I do...

I went back to my basics checked everything but I am really stumped.
Can anyone change my line of thinking ? What am I doing wrong here???

Thanks a lot,
Saran

trying to do a Sample and hold circuit?
Click to expand...
have a look at the old standby 4066 chip
it has 4 bilateral mos switches,. i used one of those
a few years ago to convert a tone from a radio
through a Voltage comparator to generate square waves
then using a low valued capacitor from the square wave
to generate a short pulse signal, i was able to control
a charging cap state that would then set a another stage
as the holding circuit..
with this, i created a Tone to Analog Voltage that would
represent the freq in a scale.
this then drove a already supplied Analog input that was there
on the computer.
the end results was a Weather Fax picture receiver...
any ways, the use of the 4066 chip gives a lot of idea's
it can operate from a single supply and does not require
- input to turn it off.


--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
 
Thanks a lot Bob and Jamie for your helpful suggestions. I figured out
the problem. Basically, as Bob said, the problem was due to the
internal diode in MOSFET. And, the 4066 data sheet was very helpful
too. Thanks.

Regards,
Saran
 
I am having the similar problem, I am trying to switch +15V using PMOS and
-15V using NMOS, gate voltage is -15V(off) and 0V(on) in case of PMOS and
+15V(on) and 0V(off) in case of NMOS. I am seeing the voltage at output
irrespective of the control gate voltage, how you get around the problem
you had?
 
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