Design Note 55: New Low Cost Differential Input Video Amplifiers Simplify Designs and Improve Performance

The LT1190 is a family of high speed amplifiers optimized for video performance on ±5V or single +5V supplies. The family includes three voltage feedback op amps and two video difference amplifiers. All amplifiers slew at 450V/μs, and deliver ±50mA output current for driving cables. The LT1193 video difference amplifier features uncommitted high input impedance (+) and (–) inputs, and can be used in differential or single-ended configurations. In addition, the LT1193 has an adjustable gain of two or greater, with a –3dB bandwidth of 80MHz.

Wideband Voltage Controlled Amplifier

The LT1193 video difference amplifier combined with an MC1496 balanced modulator make a low cost 50MHz Voltage Controlled Amplifier (VCA), shown in Figure 1. The input signal of the MC1496 at pin 1 is multiplied by the Control Voltage on Pin 10, and appears as a differential output current at Pins 6 and 12. The LT1193 acts to level shift the differential signal and convert it to a single-ended output. Resistor RB is used to set the bias current in the MC1496 to 1mA in each 200Ω RL, while RCM is used to shift the differential output into the common mode range of the LT1193. Resistors R1 through R4 bias the MC1496 inputs so that the Control Voltage VC can be referenced to 0V. Positive VC causes positive gain; negative VC gives a phase inversion (–AV), while 0V on VC gives maximum attenuation (within the VOS of the MC1496 control inputs). The value of Re is chosen by knowing the maximum input signal:

Re = (VIN max)/1mA

Figure 1. Low Cost 50MHz Voltage Controlled Amplifier.

Figure 1. Low Cost 50MHz Voltage Controlled Amplifier.

For the example shown the maximum input signal is 100mV peak, therefore, Re = 100Ω. At this maximum input signal there is significant distortion from the re modulation of the input pair. Linearity can be improved by increasing Re at the expense of gain. The maximum voltage gain of the VCA is:

Figures 2 and 3, show the frequency response and harmonic distortion of the VCA.

Figure 2. Gain and Attenuation of VCA.

Figure 2. Gain and Attenuation of VCA.

Figure 3. VCA Output Spectrum.

Figure 3. VCA Output Spectrum.

The voltage gain of the VCA can be increased at the expense of bandwidth by changing the value of load resistors RL. Shorting RCM and increasing RL to 2k will increase the maximum gain by 20dB and the –3dB bandwidth will drop to approximately 10MHz.

The LT1193 has a shutdown feature that reduces its power dissipation to only 15mW and forces a three-state output. The three-state output occurs when Pin 5 is taken to V. The high Z state, dominated by the impedance of the feedback network, is useful for multiplexing several amplifiers on the same cable. The impedance of the feedback resistors should not be raised above 1kΩ because stray capacitance on the (–) input can cause instability.

Extending the Input Range on the LT1193

Figure 4 shows a simplified schematic of the LT1193. In normal operation the REF pin 1 is grounded or taken to a DC offset control voltage, while differential signals are applied between Pins 2 and 3. The LT1193 has been optimized for gains of 2 or greater, and this means the input stage must handle fairly large input signals. The maximum input signal occurs when the input differential amplifier is tilted over hard in one direction, and 1.2mA flows through the 1kΩ Re. The maximum input swing is therefore 1.2Vp or 2.4Vp-p. The second differential pair is running at slightly larger current so that when the first input stage limits, the second stage remains biased to maintain the feedback.

Figure 4. LT1193 Simplified Schematic.

Figure 4. LT1193 Simplified Schematic.

Occasionally it is necessary to handle signals larger than 2.4Vp-p at the input. The LT1193 input stage can be tricked to handle up to 4.8Vp-p. To do this, it is necessary to ground Pin 3 and apply the differential input signal between Pin 1 and 2. The input signal is now applied across two 1k resistors in series. Since the input signal is applied to both input pairs, the first pair will run out of bias current before the second pair, causing the amplifier to go open loop. This effect is shown in Figure 5 for the amplifier operating in a closed loop gain of 1. The LT1193 has a unity gain phase margin of only 40 degrees, so when operating at unity gain, care must be taken to avoid instability.

Figure 5. LT1193 in Unity Gain (A) Standard Inputs, Pins 2 to 3, VIN = 3.6Vp-p (B) Extended Inputs, Pins 1 to 2, VIN = 3.6Vp-p (C) Extended Inputs, Pins 1 to 2, VIN = 7.0Vp-p.

Figure 5. LT1193 in Unity Gain (A) Standard Inputs, Pins 2 to 3, VIN = 3.6Vp-p (B) Extended Inputs, Pins 1 to 2, VIN = 3.6Vp-p (C) Extended Inputs, Pins 1 to 2, VIN = 7.0Vp-p.

著者

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John Wright