TYPICAL TEST CIRCUIT

The signal source in Figure 1 can be either a fast pulse gen-

erator 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.

USING THE MSK 643

The output of the amplifier is biased at one half of the power

supply voltage. An output voltage swing of ±35 volts is typi-

cal 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 of 100 to 200nH and can

best be determined by trial and error. The output of the MSK

643 is not short circuit protected, therefore, purely resistive

loads should be no less than 800 ohms at any time to avoid

damaging the output.

OPERATION CONSIDERATIONS

The input of the MSK 643 rests at a -1.55VDC level with the

input terminal open. In this state, the output rests at one half

of the power supply voltage. When connecting a pulse genera-

tor to the input of the amplifier, the DC level should be offset

so that the signal is centered around -1.55V. During character-

ization, the input should be coupled to the MSK 643 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 capaci-

tance is 8.5pF. Viewing the output with a normal oscilloscope

probe would seriously degrade performance. A FET probe fit-

ted 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.

APPLICATION NOTES

OUTPUT ISSUES

The output of the MSK 643 is a pair of bipolar emitter follow-

ers 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 varia-

tions. Connecting a wire or cable from the output of the ampli-

fier to the CRT grid can create a resonant circuit which can

cause unwanted oscillations or overshoot at its resonant fre-

quency. 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:

R = 2*

√L/C

This resistor also doubles as an arcing protector. In the bread-

boarding stage, the value of this resistor should be determined

experimentally. Resistance in the range of 50 to 100 ohms is

usually sufficient.

If a quick, simple peaking network is de-

sired, a 300 ohm cable terminated by a capacitor will act like an

inductor in the frequency range involved.

HEAT SINKING

The MSK 643 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.

WHERE

Tj = Junction Temperature

Pd = Total power dissipation

Tc = Case temperature

Ta = Ambient temperature

Ts = Sink temperature

EXAMPLE

Tj = 150°C

Ta = 100°C

Pd = 1.5W

tivity) shows that the heat sink for this application must have a

thermal resistance of no more than 6.0°C/W to maintain a junc-

tion temperature of no more than 150°C.

TRANSIMPEDANCE AMPLIFICATION

Transimpedance amplifiers relate input current to output volt-

age. The MSK 643 contains an internal 4K

Ω 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

Ω input

resistor results in an input current of ±4.77mA. This current

flows through the 4K

Ω feedback resistor and results approxi-

mately in a ±20V swing at the output. The actual voltage gain

of the typical MSK 643 circuit may be slightly less due to tran-

sistor losses. The following formula approximates voltage gain

including potential losses:

Voltage Gain (V/V) = 4K

Ω/(Rin + L)

L

≈ 25Ω

Rev. D 10/05

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