Low Power Video Buffer

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Betreff: Low Power Video Buffer
Von: g8mny@gb7cip
Gruppen: ampr.bbs.atv
Datum: 19. Mar 2016, 12:07:00
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : ATV@WW

By G8MNY                            (CQTV 1993, new to packet Feb 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

For battery use, or merely power saving in a complex vision switchers, this
circuit was development from the standard 2 transistor (25mA) design, many
existing circuits can be modified to this arrangement.

The push-pull output means a more complex design, but it enables quite
substantial reductions in power (heat) consumption (about 5 times).

FEATURES
1. Very low current drawn, just 4mA, better than some Chips?
2. Push pull output stage.
3. High Input Z >10k
4. Correctly matched Output Z, to 75
5. Low amplitude distortion, & low phase error.
6. Vision clamp, irons out poor LF response & hum
7. Vision clamp, stops sync crushing in amp.

              <+9-12V @ 4mA
                                     R8               (no HUM!)
             R3         R5                  
                                    /e    +C6
                        Ĵ TR2  ===
High Z    C1          /    C4+ R6  \     
Video o)Ĵ)ĴTR1   ===           +      75
Input          __   \e         R10)Ĵ(o Video
               /_\D1   )R7Ĵ       C5   Output
        R1    Ĵ                /          
                      C3   Ĵ TR3       
 Optional      ===C2     ===       \e         
    Term  /   R2       R4                      
                                  R9         
       <0v

Type\No  1      2      3     4     5     6     7     8     9     10
  R     75    33k   100k   2k7   2k7   220k  3k3   10   10    39
  C     u2     u1     22p   10u   470u   47u
  Tr   BC109  BC212  BC107
  D    1N4148

HOW IT WORKS
C1-2, D1, & R2-3 form the input biasing clamp, this maintains the input voltage
at 3V, so with a gain of 2x, the output voltage will always be around 6V.

TR1-2, & R5 make up the standard non inverting amplifier with the gain being
set by (R4+R7)/R4. However R8 provides extra current negative feed back on the
TR2 emitter, which reduces the loop gain slightly, but greatly improves
linearity. This consiquently enables the more linear driving voltage in T2 base
to be suitable to feed (via C4) a complementary output components TR3 & R9. TR3
provides the light DC quescent load of 3mA, from the low base current through
R6.

The HF trim is set up by C3 at a value of 22pF (try a preset if you like). This
results in no phase error (lead/lag on Vectorscope) through the amplifier. With
faster transistors, a reduced value for C3 may be required for perfect colour
phase alignment.

CONCLUSION
To give 75 output Z, R10 has to be lower than the frequently encounterd 68,
this is due to the slighty high output impedance created by the loop gain being
lowered by the linearizing effects of R8 & R9. This limits the usefulness of
this circuit for multiple outputs.
The power supply must clean, as any ripple present will be fed into the TR3
base!


Why Don't U send an interesting Bul?

73 de John G8MNY @ GB7CIP

Datum Thema#  Autor
19.03. o Low Power Video Buffer1g8mny

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