Genius 3693 Video Amplifier Circuit, Function, Description, Video Amplifier Circuit Description

VIDEO AMPLIFIER CIRCUIT, FUNCTION, DESCRIPTION.

The video amplifier, is a high speed push pull

127V

amplifier, which can swing as much as 90 volts.

120V

560Ω

The maximum dynamic output swing is limited to

18

60 volts. The rest of the output voltage range is

K17

140Ω

K14

reserved for bias adjustment.

2SA1370

+12V

12.1K

K16

Ω

2N

1.49K

K15

.015uF

VIDEO

301

3904

K9

40.2K

885

OUTPUT

K2

INTERFACE

	K34	1			K19	16V	2SC3467
			+			MPS2907
	Ω
	606		NE592		7
	K7
							15Ω
		14
						945	K4
C5346	681Ω			5.62K
1.50K				K11		Bias Control Line	+9.25V line
K1	K6	836Ω
						from Auto Bias IC
SIMPLIFIED VIDEO		K10
AMPLIFIER CIRCUIT:

The video amplifier drive circuit is built on a ceramic substrate which is a good heat sink. The printed resistors and conductors are small and have precise geometries which output a faithful reproduction of the input signal with good high frequency response and low overshoot.

The NE592 is a 120MHz emitter coupled differential amplifier which is connected to a push pull output stage. This output stage has a low bias current of 3mA and a bandwidth of 25MHz. Although at 25MHz, the output stage current increases to 15mA.

VIDEO AMPLIFIER CIRCUIT DESCRIPTION

The video amplifier's output voltage, with no input signal, is the black level which is the picture tube cut off voltage. This voltage is set, for each of the three video amplifiers, by the auto bias circuit via the bias control line. This black level voltage has a range of 80V to 110V.

The voltage swing at the output is 60 volts for a 10 mA current signal from the C5346. For this same 10 mA current signal, the voltage swing at the video amplifier input is

1.5volts and the input voltage swing at the NE592 is .80 volts. The reason for using the voltage divider K6 and K7 is that the C5346 minimum output voltage is 7.7 volts, and the bias voltage at the NE592 input is 5.3 volts.

The input signal is buffered by a NPN

transistor	K34	for low input capacitance.
Resistors		and		set the black level
	K1		K2

input voltage at 10 volts. Resistor K21 (see

schematic next page) and diode K36 protect the input against arc related transients. Resistors K6 and K7 are used to set the input bias voltage for the NE592. Diode K35 acts as a temperature compensator to match the emitter to base diode in the

buffer transistor.

The negative feedback circuit consists of

bias resistors			K9	,	K10	and output feedback
resistor		.	The geometry of the feedback
	K19

circuit defines the AC negative feedback path. The autobias output, which adjusts the black level voltage, is also connected to this node

through resistor K11 . Solder connection A connects resistor K22 , which raises the video amp. output voltage by 10V, for some CRTs.

The voltage gain of the differential amplifier K32 is set by resistor K8 . The

output of the amplifier has a load resistor K5 for faster low going transitions and is

buffered by a PNP transistor 945 with base

matching resistor K20 . The load resistor for

this buffer amplifier is K3 which is connected to the 16 volt line.

The buffered output of the differential amplifier is DC coupled to the NPN transistor

	943	and AC coupled, by capacitor						885	, to
	the	PNP transistor		of the push-pull
			951
output stage. Resistors						and
					K18		953

protect the push-pull transistors from current spikes caused by voltage transients when CRT arcing occurs.

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