Datasheet

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LT1228

1228fc

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Video DC Restore (Clamp) Circuits

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Video Differential Input Amplifiers

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Video Keyer/Fader Amplifiers

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AGC Amplifiers

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Tunable Filters

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Oscillators

100MHz Current Feedback

Amplifier with DC Gain Control

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Very Fast Transconductance Amplifier

Bandwidth: 75MHz

= 10 × I

SET

Low THD: 0.2% at 30mV

RMS

Input

Wide I

SET

Range: 1µA to 1mA

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Very Fast Current Feedback Amplifier

Bandwidth: 100MHz

Slew Rate: 1000V/µs

Output Drive Current: 30mA

Differential Gain: 0.04%

Differential Phase: 0.1°

High Input Impedance: 25MΩ, 6pF

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Wide Supply Range: ±2V to ±15V

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Inputs Common Mode to Within 1.5V of Supplies

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Outputs Swing Within 0.8V of Supplies

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Supply Current: 7mA

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Available in 8-Lead PDIP and SOIC Packages

The LT

1228 makes it easy to electronically control the

gain of signals from DC to video frequencies. The LT1228

implements gain control with a transconductance ampli-

fier (voltage to current) whose gain is proportional to an

externally controlled current. A resistor is typically used to

convert the output current to a voltage, which is then

amplified with a current feedback amplifier. The LT1228

combines both amplifiers into an 8-pin package, and

operates on any supply voltage from 4V (±2V) to 30V

(±15V). A complete differential input, gain controlled

amplifier can be implemented with the LT1228 and just a

few resistors.

The LT1228 transconductance amplifier has a high imped-

ance differential input and a current source output with wide

output voltage compliance. The transconductance, g

, is

set by the current that flows into Pin 5, I

SET

. The small signal

is equal to ten times the value of I

SET

and this relationship

holds over several decades of set current. The voltage at Pin

5 is two diode drops above the negative supply, Pin 4.

The LT1228 current feedback amplifier has very high input

impedance and therefore it is an excellent buffer for the

output of the transconductance amplifier. The current feed-

back amplifier maintains its wide bandwidth over a wide

range of voltage gains making it easy to interface the

transconductance amplifier output to other circuitry. The

current feedback amplifier is designed to drive low imped-

ance loads, such as cables, with excellent linearity at high

frequencies.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Frequency Response

FREQUENCY (Hz)

100k

–24

GAIN (dB)

–15

–3

1M 10M 100M

LT1228 • TA02

–6

–9

–12

–18

–21

SET

= 100µA

= ±15V

= 100Ω

SET

= 1mA

SET

= 300µA

LT1228 • TA01

–

R3A

10k

R2A

10k

100Ω

4.7µF

1.24k

6.19k

10k

SET

270Ω

10Ω

470Ω

4.7µF

15V

CFA V

OUT

–15V

–

HIGH INPUT RESISTANCE

EVEN WHEN POWER IS OFF

–18dB < GAIN < 2dB

≤ 3V

RMS

Differential Input Variable Gain Amp

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Summary of content (20 pages)

PAGE 1
LT1228 100MHz Current Feedback Amplifier with DC Gain Control U FEATURES DESCRIPTIO ■ The LT®1228 makes it easy to electronically control the gain of signals from DC to video frequencies. The LT1228 implements gain control with a transconductance amplifier (voltage to current) whose gain is proportional to an externally controlled current. A resistor is typically used to convert the output current to a voltage, which is then amplified with a current feedback amplifier.
PAGE 2
LT1228 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) ORDER PART NUMBER LT1228CN8 LT1228CS8 LT1228IN8 LT1228IS8 S8 PART MARKING TOP VIEW Supply Voltage ...................................................... ±18V Input Current, Pins 1, 2, 3, 5, 8 (Note 8) ............ ±15mA Output Short Circuit Duration (Note 2) ......... Continuous Operating Temperature Range LT1228C ................................................ 0°C to 70°C LT1228I ......................................
PAGE 3
LT1228 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Current Feedback Amplifier, Pins 1, 6, 8. ±5V ≤ VS ≤ ±15V, ISET = 0µA, VCM = 0V unless otherwise noted. SYMBOL PARAMETER CONDITIONS Inverting Input Current Common Mode Rejection VS = ±15V, VCM = ±13V, TA = 25°C VS = ±15V, VCM = ±12V VS = ±5V, VCM = ±3V, TA = 25°C VS = ±5V, VCM = ±2V MIN TYP MAX UNITS 2.
PAGE 4
LT1228 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. Transconductance Amplifier, Pins 1, 2, 3, 5. ±5V ≤ VS ≤ ±15V, ISET = 100µA, VCM = 0V unless otherwise noted. SYMBOL PARAMETER CONDITIONS IB Input Bias Current TA = 25°C MIN TYP 0.
PAGE 5
LT1228 U W TYPICAL PERFOR A CE CHARACTERISTICS Transconductance Amplifier, Pins 1, 2, 3 & 5 100 VS = ±15V Small-Signal Transconductance and Set Current vs Bias Voltage 100 R1 = 100Ω TRANSCONDUCTANCE (µA/mV) 10 R1 = 10k 1 10000 VS = ±2V TO ±15V TA = 25°C 2.0 1000 10 100 1 0.1 10 0.01 1.0 R1 = 100k 100 1000 0.001 0.9 1.0 SET CURRENT (µA) 1.1 1.2 0.1 1.5 25°C 0.8 0.6 125°C 0.4 0 –200 –150 –100 –50 0.
PAGE 6
LT1228 U W TYPICAL PERFOR A CE CHARACTERISTICSCurrent Feedback Amplifier, Pins 1, 6, 8 Voltage Gain and Phase vs Frequency, Gain = 6dB 160 6 90 140 GAIN 135 4 180 3 225 2 VS = ±15V RL = 100Ω RF = 750Ω RF = 500Ω 120 RF = 750Ω 100 80 RF = 1k 60 40 RF = 2k 20 –1 –2 0.
PAGE 7
LT1228 U W TYPICAL PERFOR A CE CHARACTERISTICS Current Feedback Amplifier, Pins 1, 6, 8 Maximum Capacitive Load vs Feedback Resistor Total Harmonic Distortion vs Frequency 0.10 VS = ±15V 100 RL = 1k PEAKING ≤ 5dB GAIN = 2 10 1 0.01 VO = 7VRMS 2 1 10 100 1k FEEDBACK RESISTOR (kΩ) 10k –1.5 –2.0 2.0 V – = –2V TO –15V 1.0 0.5 0 25 50 75 100 Output Short-Circuit Current vs Temperature 70 –0.5 –1.0 RL = ∞ ±2V ≤ VS ≤ ±15V 1.0 0.
PAGE 8
LT1228 U W TYPICAL PERFOR A CE CHARACTERISTICS Current Feedback Amplifier, Pins 1, 6 & 8 Settling Time to 10mV vs Output Step Settling Time to 1mV vs Output Step INVERTING 4 4 OUTPUT STEP (V) 6 VS = ±15V RF = RG = 1k 0 NONINVERTING 8 6 2 10 –2 –4 9 8 INVERTING SUPPLY CURRENT (mA) NONINVERTING 8 OUTPUT STEP (V) Supply Current vs Supply Voltage 10 10 2 VS = ±15V RF = RG = 1k 0 –2 –4 INVERTING –6 –6 –8 NONINVERTING NONINVERTING 0 20 40 60 80 100 5 125°C 4 175°C 3 1 –1
PAGE 9
LT1228 U W U UO APPLICATI S I FOR ATIO The LT1228 contains two amplifiers, a transconductance amplifier (voltage-to-current) and a current feedback amplifier (voltage-to-voltage). The gain of the transconductance amplifier is proportional to the current that is externally programmed into Pin 5. Both amplifiers are designed to operate on almost any available supply voltage from 4V (±2V) to 30V (±15V).
PAGE 10
LT1228 W U U UO APPLICATI S I FOR ATIO Substituting into the equation for transconductance gives: gm = a 10 = 1.94R R The temperature variation in the term “a” can be ignored since it is much less than that of the term “T” in the equation for Vbe. Using a 2.5V source this way will maintain the gain constant within 1% over the full temperature range of –55°C to 125°C. If the 2.5V source is off by 10%, the gain will vary only about ±6% over the same temperature range.
PAGE 11
LT1228 U W U UO APPLICATI S I FOR ATIO compliance compatible with Pin 5 of the LT1228. Therefore, the easiest way to digitally control the set current is to use a voltage output DAC and a voltage-to-current circuit. The previous voltage-to-current converter will take the output of any voltage output DAC and drive Pin 5 with a proportional current. The R, 2R CMOS multiplying DACs operating in the voltage switching mode work well on both single and split supplies with the above circuit.
PAGE 12
LT1228 U W U U APPLICATIO S I FOR ATIO the family of curves that result from various values of the feedback resistor. These curves use a solid line when the response has less than 0.5dB of peaking and a dashed line for the response with 0.5dB to 5dB of peaking. The curves stop where the response has more than 5dB of peaking.
PAGE 13
LT1228 U W U UO APPLICATI S I FOR ATIO Capacitive Loads The LT1228 current feedback amplifier can drive capacitive loads directly when the proper value of feedback resistor is used. The graph of Maximum Capacitive Load vs Feedback Resistor should be used to select the appropriate value. The value shown is for 5dB peaking when driving a 1k load, at a gain of 2. This is a worst case condition, the amplifier is more stable at higher gains, and driving heavier loads.
PAGE 14
LT1228 U W U U APPLICATIO S I FOR ATIO Power Supplies The LT1228 amplifiers will operate from single or split supplies from ±2V (4V total) to ±18V (36V total). It is not necessary to use equal value split supplies, however the offset voltage and inverting input bias current of the current feedback amplifier will degrade. The offset voltage changes about 350µV/V of supply mismatch, the inverting bias current changes about 2.5µA/V of supply mismatch.
PAGE 15
LT1228 U TYPICAL APPLICATIO S Video Fader Video DC Restore (Clamp) Circuit NOT NECESSARY IF THE SOURCE RESISTANCE IS LESS THAN 50Ω VIN1 1k 3 + 1 gm 2 1000pF LT1223 CFA – 5 100Ω + 200Ω 3 VOUT V+ 7 + – 1k – 10k –5V 10k CFA 8 VOUT – RF V– 5.1k 6 10k 5.1k VS = ±5V + 0.
PAGE 16
LT1228 U TYPICAL APPLICATIO S Single Supply Wien Bridge Oscillator resistor and the transconductance amplifier must be about 11, resulting in a set current of about 600µA at oscillation. At start-up there is no set current and therefore no attenuation for a net gain of about 11 around the loop. As the output oscillation builds up it turns on the PNP transistor which generates the set current to regulate the output voltage.
PAGE 17
LT1228 U TYPICAL APPLICATIO S Filters Single Pole Low/High/Allpass Filter R3A 1k VIN LOWPASS INPUT 3 + 1 gm R3 120Ω 2 – 5 C 330pF ISET + CFA 8 VOUT – RF 1k RG 1k VIN HIGHPASS INPUT 6 R2A 1k R2 120Ω fC = R +1 R2 10 I × SET × F × C RG R2 + R2A 2π fC = 109 ISET FOR THE VALUES SHOWN LT1228 • TA16 Allpass Filter Phase Response PHASE SHIFT (DEGREES) 0 1mA SET CURRENT –45 –90 –135 100µA SET CURRENT –180 10k 100k 1M 10M FREQUENCY (Hz) LT1228 • TA17 Using the variable transconduct
PAGE 18
LT1228 U TYPICAL APPLICATIO S Voltage Controlled State Variable Filter + 1k LT1006 10k – 2N3906 VC 100pF 180Ω 51k 3k –5V 5V 3.3k 3 VIN 3k 7 + 100Ω 5 1 gm 2 – + 6 CFA 4 8 18pF BANDPASS OUTPUT – –5V 1k 3.3k 3.3k 100Ω 5V 3 7 + 5 gm 100Ω 2 – 1 + 6 CFA 4 18pF –5V 8 LOWPASS OUTPUT – 1k 3.3k fO = 100kHz AT VC = 0V fO = 200kHz AT VC = 1V fO = 400kHz AT VC = 2V fO = 800kHz AT VC = 3V fO = 1.6MHz AT VC = 4V The state variable filter has both lowpass and bandpass outputs.
PAGE 19
LT1228 U PACKAGE DESCRIPTIO J8 Package 8-Lead CERDIP (Narrow .300 Inch, Hermetic) (Reference LTC DWG # 05-08-1110) CORNER LEADS OPTION (4 PLCS) .300 BSC (7.62 BSC) .023 – .045 (0.584 – 1.143) HALF LEAD OPTION .008 – .018 (0.203 – 0.457) 0° – 15° .405 (10.287) MAX .005 (0.127) MIN .045 – .068 (1.143 – 1.650) FULL LEAD OPTION 8 6 7 .200 (5.080) MAX 5 .025 (0.635) RAD TYP .015 – .060 (0.381 – 1.524) .220 – .310 (5.588 – 7.874) 1 2 3 4 .045 – .065 (1.143 – 1.
PAGE 20
LT1228 U TYPICAL APPLICATIO S RF AGC Amplifier (Leveling Loop) 15V RF INPUT 0.6VRMS to 1.3VRMS 25MHz 10k 3 7 + 1 gm 100Ω 2 – + 5 OUTPUT 2VP–P CFA 300Ω 8 – 4 470Ω 0.01µF 10k –15V 10k 4pF 10Ω 0.01µF 10k 15V – 10k 100k 4.7k –15V A3 LT1006 AMPLITUDE ADJUST + 1N4148’s COUPLE THERMALLY LT1004 1.2V –15V LT1228 • TA20 Inverting Amplifier with DC Output Less Than 5mV Amplitude Modulator 5V 2 7 – 3 1 gm 3 + 5 + 4 4.