WIDE BANDWIDTH, VERY HIGH VOLTAGE CRT VIDEO AMPLIFIER
4707 Dey Road Liverpool, N.Y. 13088 Pin Compatible with LH3424 and CR3424 High-Rel Versions 2.5nS Transition Times Drives 8.5pF Capacitive Load With Ease DC Coupled for Output Level Adjust 175MHz Bandwidth 75Vpp Output Swing
The MSK is a wide bandwidth, high voltage color or monochrome CRT video amplifier designed specifically to drive the cathode of today's most demanding high resolution CRT monitors. The MSK is a transimpedance amplifier capable of achieving a �40V output voltage swing with an input current of �10mA. The output of the amplifier is DC biased at half the power supply voltage. Transition times in the range of 2.5nS enable the MSK 641 to drive 10nS pixels with ease and make it ideally suited for monitors with 1024 or higher display resolutions. The 9 pin single in-line bathtub package is pin for pin compatible with the LH3424 and CR3424 and is a drop in replacement for the high-rel versions of these devices with improved stability and thermal performance.TYPICAL APPLICATIONS
CRT Driver for Color and Monochrome Monitors High Voltage Transimpedance Amplifier Ultra High Speed Amplifier for Test Equipment+VCC JC IOUT Supply Voltage Thermal Resistance (Junction to Case) Peak Output Current
Storage Temperature Range to +150�C Lead Temperature Range 300�C (10 Seconds) Case Operating Temperature to +125�C Junction Temperature 175�C
Parameter STATIC Power Supply Current VIN=N/C 2 3 Input Bias Voltage Output Offset Voltage Input Capacitance 2 Power Supply Range DYNAMIC CHARACTERISTICS Output Voltage High Output Voltage Low Voltage Gain Rise Time Fall Time Overshoot (Adjustable) 2 -3dB Bandwidth Linearity Error
CIN=100pF, CLOAD=8.5pF, RL=, unless otherwise specified (See Figure 1). Guaranteed by design but not tested. Typical parameters are representative of actual device performance but are for reference only. Industrial grade devices shall be tested to subgroups 1 and 4 unless otherwise specified. Military grade devices ('B' suffix) shall be 100% tested to subgroups 1,2,3 and 4. Subgroup 5 and 6 testing available upon request. Subgroup 1,4 TA=TC=+25�C Subgroup 2,5 TA=TC=+125�C Subgroup 3,6 TA=TC=-55�C
The signal source in Figure 1 can be either a fast pulse generator or a network analyzer as long as the output impedance is 50 ohms. The DC level of the input should be 1.55V and all cables should be kept as short as possible. Since total load capacitance should be kept below 8.5pF, a FET probe should be used on the ouput.
The output of the MSK is a pair of bipolar emitter followers configured in a complimentary push pull configuration. This configuration eliminates the need for a pull up load resistor and makes the amplifier less susceptible to load capacitance variations. Connecting a wire or cable from the output of the amplifier to the CRT cathode can create a resonant circuit which can cause unwanted oscillations or overshoot at its resonant frequency. A damping resistor in series with the lead inductance will alleviate this condition. The optimum value of this resistor can be determined using the following formula: = 2* L/C This resistor also doubles as an arcing protector. In the breadboarding stage, the value of this resistor should be determined experimentally. Resistance in the range to 100 ohms is usually sufficient. If a quick, simple peaking network is desired, a 300 ohm cable terminated by a capacitor will act like an inductor in the frequency range involved.
The output of the amplifier is biased at one half of the power supply voltage. An output voltage swing of �35 volts is typical with a power supply voltage of +80 volts. With an 8.5pF capacitive load, transistion times are in the 2.5nS range. If a spark gap current limiting resistor is used on the output of the amplifier and the transistion times are degraded, a peaking coil may be used to preserve system performance. The optimum value for this coil will be in the range to 200nH and can best be determined by trial and error. The output of the MSK 641 is not short circuit protected, therefore, purely resistive loads should be no less than 800 ohms at any time to avoid damaging the output.
Transimpedance amplifiers relate input current to output voltage. The MSK 641 contains an internal 4K feedback resistor. This resistor converts input current to output voltage in the following manner (See Figure 1): �1.43V (referenced to 1.55Vdc) across the 300 input resistor results in an input current of �4.77mA. This current flows through the 4K feedback resistor and results approximately a �20V swing at the output. The actual voltage gain of the typical MSK641 circuit may be slightly less due to transistor losses. The following formula approximates voltage gain including potential losses: Voltage Gain (V/V) L 25
The input of the MSK 641 rests a +1.55VDC level with Vcc=+80VDC and the input terminal open. In this state, the output rests at one half of the power supply voltage. When connecting a pulse generator to the input of the amplifier, the DC level should be offset so that the signal is centered around +1.55V. During characterization, the input should be coupled to the MSK 641 through a parallel combination of a variable resistor and variable capacitor peaking circuit. Optimum values for the peaking circuit can be determined experimentally. The optimum value of load capacitance is 8.5pF. Viewing the output with a normal oscilloscope probe would seriously degrade performance. A FET probe fitted with a 100:1 voltage divider will add only approximately 1.5pF of capacitance to the load and is highly recommended. An experimental circuit along with recommended values can be found in Figure 2.
The MSK 641 requires heat sinking in most applications. The following formula may be applied to determine if a heat sink is necessary and what size and type to use. Rsa = ((Tj-Ta)/Pd ) - (Rjc) - (Rcs) WHERE Tj = Junction Temperature Pd = Total power dissipation Rjc = Junction to case thermal resistance Rcs = Case to heat sink thermal resistance Rsa = Heat sink to ambient thermal resistance Tc = Case temperature Ta = Ambient temperature Ts = Sink temperature EXAMPLE = 1.5W Rjc = 27�C/W Rcs = 0.15�C/W Solving the above equation for Rsa (heat sink thermal conductivity) shows that the heat sink for this application must have a thermal resistance of no more than 6.0�C/W to maintain a junction temperature of no more than 150�C. Rev. 6/02 3