Oops schematic mistake on first message. Correction made --> Help. Transistor circuit that connect

I breadboard separate two transistor circuits to compares the
performance of switching in and out a 75 ohm resistor terminator to a
video source. One circuit is based on a bi-polar 2N3904 and the other
is a FET transistor(it's a DMOS N-channel MOSFET). Using a video
generator that is set to sweep 1 to 30MHz and an oscilloscope to
monitor the response, I observed that both type of terminators have
issues. On the bipolar method, the frequency response is pretty flat
to 30MHz when the 2n3904 is truned off, but when it is turned on, the
frequency response has gain peaks and loss valleys. This cause the
video sync to measure 40IRE (that's good), but the color burst
measures 48IRE (too high). I need the frequency response to be flat to
10 MHz. The FET mehtod has the reverse effect. When the FET is on,
the frequency response is pretty flat to 30MHz. When the FET is OFF,
the frequency response has a number of gain peaks and loss valleys.
Also, on both method, the video signal wobbles slowly slightly up and
down (on or foo on both methods). I can't seem to get rid of the
wobble. Must be due to the stray capacitance of both FET and bipolar.
Any help will be appreciated.


Circuit of both termination method:


video source------------------*-------------------Oscilloscope
|
|
75 ohm
|
|
C
0V or 5VDC-->10k-------->B 2N3904
E
|
|
-5VDC


video source------------------*-------------------Oscilloscope
|
|
75 ohm
|
D
0V or 5VDC-->10k-------->G N-channel DMOS FET RON(MAX)=2ohm
S for VGS= 5V
|
| |
|
-5VDC

 

Thanks John for you feedback,

The base to emitter is always forward bais regardless whether the base
is 5v or 0v. True. The picture showing 0v is a mistake, since by
turning off the lab power supply I really disconnected the base with
no forward bais to the BE section. But the -5v is required to the
emitter of the transistor since the video source or generator that I
am using is AC coupled and the video generator that I am using is a
video standard. The generator unfortunately does not allow DC baising
and I want to keep the circuit on the T connector simple. On the
bipolar, all I want to really see, how's the frequency response looks
lke with the bipolar is on. What I really want to use is the FET
design unless there is a strong argument in favor of the bi-polar.
The FET is better in my mind for an AC coupled signal. On the FET
diagram, the -5 to the source is really on the setup connected to 0v.
For now, I am really concerned with the frequency response and the
slow vertical motion that I see on the video scope analyzer. The
vertical slow motion of the video signal tells me that the 75ohm
resistor is not connected to ground effectively through thr FET.
Also, on the peaks and valleys of the 1 to 30 MHz frequency response,
as was replied by another helpful responder, FET switch/75 ohm
resistor design probably make a poor terminator for a video 75 ohm
coax line. The FET has all sort of stray capacitance with its small
RDS resistance that is probably causing the frequency issues. Other
vendor who sell video products with a swithable load to the 75
terminator use a relay. But I spoke to a video expert in the field
(MAXIM) and he design a workable solid state switchable load for video
line termination. I am assuming he used a FET switch to the 75 ohm
resistor, but there is probably more to this simplied design. Any
thoughts would be appreciated.


Caesar

 

(view with a fixed-pitch font)

this one may work a bit better

--+--- +5
|
10K
source | 2N3906
| 1N914 |
| +-->|--+-------C E-------+---- -5
| | | B |
\ 150R === 100pF |---[33k]-'
\ / | 1K .---[10K]-- control
X --+-- 0V | .2N3906 | (0 or 5V)
/ \ | C ' B
/ 150R === 100pF B---[10K]--+-C E-- 0V
| | | E | `.
| | | |--[33K]-. 47K 2N33904
| | | B | |
| +--|<--+-------C E------+--+--- +5
| 1N914 |
scope | 2N3904
10K
|
--+--- -5

TTL-swittched broadband terminator concept.


Bye.
Jasen

 

Jasen,

Thank you for your response. I am having trouble reading your ascii
schematic. Also "Fixed font" I don't understand. I am excited to
understand your circuit. You have to excuse me since I am new in
reading ascii schematic. As you can see my ascii schematic is not
very good. Is there any way to send an attachment with a picture,
pdf or help me understand your ascii schematic?
Luis

 

Disregard previous message. The key is as you said fixed pitched such
as new courier. Everything lines up. Thanks.

Caesar

 

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar

 

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar

 

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar

 

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar

 

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar

 

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked
at it, I like the idea how D1 and D2 are used to terminate through the
voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then
reverse D1 and D2 to no termination. Very clever. I ran this circuit
on the simulator and toggle J1 between 5v and 0v. Unfortunately, I
saw no change on the right side of R11. Checking on the circuit I
notice that Q5 and Q1 can never turn on regardless if the right side
of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets
forward bais to turn the transistor. The same is Q5. I can't seem to
see any bipolar configuration for Q2 to switch Q5 and Q1 on or off.
Can you see if I redrew your schematic correct. I hope to get your
schematic to work on Workbench first before I try to bread-board it.
Your idea for the diode to switch on or off the termination make
sense. I just have to get the logic controls to work proper. Please
send me your feedback.

Caesar

 

Set whatever you are using for a news reader to use Courier font.