Video amplifier with adjustable gain
This one is a video amplfier which is a part of a video amplifer form one RGB adapter.
o +5v
|
+------+-----------+
| | |
1N4148 | |P1 | | R2 |
+-|<|->| |5k | | 1k |
| |_| |_| | T2
| | | b|<|e BC560
| --- +---------|
Video in | b|/ c T1 |\ c ___ R3
>---+--||-+------------| BC550 +----|___|------> to black level clamping
| 1 uF |>|e | 75 ohm 2Vp-p
| | | | | P2
| | R1 +--------->| | 5 k potentiometer
|_| 75 ohm |_|
| |
--- ---
The circuit has some adjustments: P1 adjusts the DC level in the video signal and P2 adjuts the amplification of the circuit (output level). The circuit is designed to handle 1 Vpp input signals and has enough bandwidth to handle even VGA signals (around 30 MHz).
The input impedance is 75 ohm, but it can be made high impedance by removing R1. The output impedance is 75 ohm so it is suitable for driving long coaxial cables if necessary.
How to add Sync to video signal
This is a circuit which can be used to add the sync signal to existing vidoe signal which does not have sync. For example when sync has to been added to Green video signal for for making Sync-on-Green signal.
o +5v
|
+------+-----------+
| | |
1N4148 | |P1 | | R2 |
+-|<|->| |5k | | 1k |
| |_| |_| | T2
| | | b|<|e BC560
| --- +---------|
Video in | b|/ c T1 |\ c ___ R3
<---+--||-+------------| BC550 +----|___|------> to black level clamping
| 1 uF |>|e | 75 ohm 2Vp-p
| | | | | P2
| | R1 +--------->| | 5 k potentiometer
|_| 75 ohm |_|
| |
--- |
|
TTL level CSYNC in >-------------+ |
| |
P3 | | |
250 ohm | |<-+
|_|
|
---
The circuit has some adjustments: P1 adjusts the DC level in the video signal, P2
adjusts the amplification of the circuit (output level) and P3 sync signal level.
Simple video buffer
This is a simpel video buffer curcuit. It is DC coupled circuit to be used for example as video output buffer.
o +5v
|
+----+
| |
R1 | | |
1k | | |
|_| | T2
| b|/ c BC547
+--|
| |>|e ___ R3
b|<|e +----|___|-------> video out
in >---| | 75 ohm 1Vpp @ 75 ohm
T1 |\ | | R2 +-->
BC557 | | | 1k |
| |_| |
| | ---
+----+
|
---
The amplification of the transistor part of the circuit is 1 and
the output DC level is about the same as the DC level of the input.
Because there is 75 ohm resistor in output to match the output impedance
you must send double the level to the circuit you want to send to a 75 ohm
terminated load.
o +5v
|
+---+
| |
1k R |
| |c BC108B
+-b
2Vpp | |e 75ohm
video |e +--------R-------> video out
in >---b | 1Vpp @ 75 ohm
|c R 1k +-->
BC178B| | |
+---+ ---
|
o -5v (yes, two-sided power supply, not ground)
Information sources
- Video amplifier by Mika Iisakkila
- RGB video amplifier, Elektor Electronics July/August 1997, pages 89-90
Tomi Engdahl <[email protected]>