EL2020CN Просмотр технического описания (PDF) - Elantec -> Intersil

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EL2020CN

EL2020C / 50 MHz Current Feedback Amplifier

Elantec -> Intersil 

EL2020C

50 MHz Current Feedback Amplifier

Application Information Contd

Summing Amplifier

EL2020 Typical Inverting

Amplifier Characteristics

2020 – 12

10V

AV RF R1 R2 Bandwidth Settling Time

1%

0 1%

b1 750X

b2 750X

b5 680X

b10 680X

750X

375X

130X

68X

40 MHz

40 MHz

40 MHz

30 MHz

50 ns

55 ns

55 ns

70 ns

130 ns

160 ns

160 ns

170 ns

Input Range

The non-inverting input to the EL2020 looks like

a high resistance in parallel with a few picofarads

in addition to a DC bias current The input char-

acteristics change very little with output loading

even when the amplifier is in current limit

The input charactersitics also change when the

input voltage exceeds either supply by 0 5V This

happens because the input transistor’s base-col-

lector junctions forward bias If the input exceeds

the supply by LESS than 0 5V and then returns

to the normal input range the output will recov-

er in less than 10 ns However if the input ex-

ceeds the supply by MORE than 0 5V the recov-

ery time can be 100’s of nanoseconds For this

reason it is recommended that Schottky diode

clamps from input to supply be used if a fast re-

covery from large input overloads is required

Source Impedance

The EL2020 is fairly tolerant of variations in

source impedances Capacitive sources cause no

problems at all resistive sources up to 100 kX

present no problems as long as care is used in

board layout to minimize output to input cou-

pling Inductive sources may cause oscillations a

1 kX resistor in series with the input lead will

usually eliminate problems without sacrificing

too much speed

Current Limit

The EL2020 has internal current limits that pro-

tect the output transistors The current limit

goes down with junction temperature rise At a

junction temperature of a175 C the current lim-

its are at about 50 mA If the EL2020 output is

shorted to ground when operating on g15V sup-

plies the power dissipation could be as great as

1 1W A heat sink is required in order for the

EL2020 to survive an indefinite short Recovery

time to come out of current limit is about 50 ns

Using the 2020 with Output Buffers

When more output current is required a wide-

band buffer amplifier can be included in the feed-

back loop of the EL2020 With the EL2003 the

subsystem overshoots about 10% due to the

phase lag of the EL2003 With the EL2004 in the

loop the overshoot is less than 2% For even

more output current several buffers can be paral-

leled

EL2020 Buffered with an EL2004

2020 – 13

Capacitive Loads

The EL2020 is like most high speed feedback am-

plifiers in that it does not like capacitive loads

between 50 pF and 1000 pF The output resist-

ance works with the capacitive load to form a

second non-dominate pole in the loop This re-

sults in excessive peaking and overshoot and can

lead to oscillations Standard resistive isolation

techniques used with other op amps work well to

isolate capacitive loads from the EL2020

11

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