LT1228: 100MHz Current Feedback Amplifier with DC Gain Control

Features

Very Fast Transconductance Amplifier: Bandwidth: 75MHz, gm = 10 × ISET, Low THD: 0.2% at 30mVrms Input, Wide ISET Range: 1µA to 1mA
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
Wide Supply Range: ±2V to ±15V
Inputs Common Mode to Within 1.5V of Supplies
Outputs Swing Within 0.8V of Supplies
Supply Current: 7mA
Available in 8-Lead PDIP and SO Packages

Applications

Video DC Restore (Clamp) Circuits
Video Differential Input Amplifiers
Video Keyer/Fader Amplifiers
AGC Amplifiers
Tunable Filters
Oscillators

Description

The LT1228 simplifies the control of signal gain from DC to video frequencies. It integrates a transconductance amplifier (voltage-to-current) with a gain proportional to an externally controlled current, and a current feedback amplifier (voltage-to-voltage). The LT1228 operates on supply voltages from ±2V to ±15V. A complete differential input, gain-controlled amplifier can be implemented with the LT1228 and a few resistors. The transconductance (gm) is set by the current at Pin 5 (ISET), and is equal to 10 times ISET. The current feedback amplifier provides a high impedance buffer for the transconductance amplifier's output, maintaining wide bandwidth across various voltage gains. It is designed to drive low impedance loads like cables with excellent linearity at high frequencies.

Typical Application: Differential Input Variable Gain Amp

This circuit demonstrates a differential input variable gain amplifier. The transconductance amplifier's gain is controlled by the ISET current. The output current is converted to a voltage and then amplified by the current feedback amplifier. The frequency response plot shows the bandwidth for different ISET values.

Absolute Maximum Ratings

Parameter	Value
Supply Voltage	±18V
Input Current, Pins 1, 2, 3, 5, 8	±15mA
Output Short Circuit Duration	Continuous
Operating Temperature Range (LT1228C)	0°C to 70°C
Operating Temperature Range (LT1228I)	-40°C to 85°C
LT1228M (Obsolete)	-55°C to 125°C
Storage Temperature Range	-65°C to 150°C
Junction Temperature (Plastic Package)	150°C
Junction Temperature (Ceramic Package - Obsolete)	175°C
Lead Temperature (Soldering, 10 sec)	300°C

Order Information

Lead Free Finish	Tape and Reel	Part Marking	Package Description	Temperature Range
LT1228CN8#PBF	LT1228CN8#TRPBF	LT1228CN8	8-Lead Plastic DIP	0°C to 70°C
LT1228IN8#PBF	LT1228IN8#TRPBF	LT1228IN8	8-Lead Plastic DIP	-40°C to 85°C
LT1228CS8#PBF	LT1228CS8#TRPBF	1228	8-Lead Plastic SO	0°C to 70°C
LT1228IS8#PBF	LT1228IS8#TRPBF	12281	8-Lead Plastic SO	-40°C to 85°C
LT1228MJ8	LT1228MJ8#TRPBF	LT1228MJ8	8-Lead CERDIP	-55°C to 125°C
LT1228CJ8	LT1228CJ8#TRPBF	LT1228CJ8	8-Lead CERDIP	0°C to 70°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on nonstandard lead based finish parts.

Electrical Characteristics

The table below details the electrical characteristics of the LT1228 over its operating temperature range. Specifications apply at T_A = 25°C unless otherwise noted.

Symbol	Parameter	Conditions	Min	Typ	Max	Units
V_OS	Input Offset Voltage	T_A = 25°C		±0.5	±5	mV
				±10		mV
	Input Offset Voltage Drift			10		µV/°C
I_IN+	Noninverting Input Current	T_A = 25°C		±0.3	±3	µA
I_IN-	Inverting Input Current	T_A = 25°C		±10		µA
e_n	Input Noise Voltage Density	f = 1kHz, R_F = 1k, R_G = 10Ω, R_S = 0Ω		6		nV/√Hz
i_n	Input Noise Current Density	f = 1kHz, R_F = 1k, R_G = 10Ω, R_S = 10k		1.4		pA/√Hz
R_IN	Input Resistance	V_IN = ±13V, V_S = ±15V	2	25		MΩ
C_IN	Input Capacitance	V_S = ±5V		6		pF
V_CMR	Input Voltage Range	V_S = ±15V, T_A = 25°C		+13	±13.5	V
		V_S = ±5V, T_A = 25°C		+3	±3.5	V
CMRR	Common Mode Rejection Ratio	V_S = ±15V, V_CM = ±13V, T_A = 25°C	55	69		dB
I_IN	Inverting Input Current Common Mode Rejection	V_S = ±15V, V_CM = ±13V, T_A = 25°C	2.5	10		µA/V
PSRR	Power Supply Rejection Ratio	V_S = ±2V to ±15V, T_A = 25°C	60	80		dB
A_VL	Large-Signal Voltage Gain	V_S = ±15V, V_OUT = ±10V, R_LOAD = 1k	55	65		dB
R_OL	Transresistance	V_OUT/ΔI_IN	V_S = ±15V, V_OUT = ±10V, R_LOAD = 1k	100	200		kΩ
V_OUT	Maximum Output Voltage Swing	V_S = ±15V, R_LOAD = 400Ω, T_A = 25°C		+12	±13.5	V
I_OUT	Maximum Output Current	R_LOAD = 0Ω, T_A = 25°C	30	65	125	mA
I_S	Supply Current			6	11	mA
SR	Slew Rate	T_A = 25°C	300	500		V/µs
t_r	Rise Time	T_A = 25°C		10	20	ns
BW	Small-Signal Bandwidth	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 100Ω		100		MHz
t_r	Small-Signal Rise Time	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 100Ω		3.5		ns
t_pd	Propagation Delay	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 100Ω		3.5		ns
	Small-Signal Overshoot	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 100Ω		15		%
t_s	Settling Time	0.1%, V_OUT = 10V, R_F = 1k, R_G = 1k, R_L = 1k		45		ns
	Differential Gain	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 1k		0.01		%
	Differential Phase	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 1k		0.01		DEG
	Differential Gain	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 150Ω		0.04		%
	Differential Phase	V_S = ±15V, R_F = 750Ω, R_G = 750Ω, R_L = 150Ω		0.1		DEG

Package Description

The LT1228 is available in 8-Lead PDIP (N Package) and 8-Lead Plastic Small Outline (S8 Package). The J8 Package (3-Lead CERDIP) is obsolete.

Revision History

Rev	Date	Description	Page Number
D	06/12	Updated Order Information table to new format	2
		Clarified units used in gm = 10 • ISET relationship	9

Related Parts

Part Number	Description	Comments
LT1227	140MHz Current Feedback Amplifier	1100V/µs Slew Rate, 0.01% Differential Gain, 0.03% Differential Phase
LT1251/LT1256	40MHz Video Fader	Accurate Linear Gain Control: ±1% Typ, ±3% Max
LT1399	400MHz Current Feedback Amplifier	800V/µs Slew Rate, 80mA Output Current

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