Wide Range Digitally Controlled Variable Gain Amplifier

The circuit uses the LTC1043 in a variable gain amplifier which features continuously variable gain, gain stability of 20 PPM/degree C, and single-ended or differential inputs. The circuit uses two separate LTC1043s. LTC1043B is continuously clocked by a 1-kHz source, which could also be processor supplied.
Both LTC1043s function as the sampled data equivalent of a resistor within the bandwidth set by A1's 0.01-uF value and the switched-capacitor equivalent feedback resistor. The time-averaged current delivered to the summing point by LTC1043A is a function of the 0.01uF capacitor's input-derived voltage and the commutation frequency at pin 16. Low-commutation frequencies result in small time-averaged current values, and require a large impute resistor. Higher frequencies require an equivalent small input resistor.

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Wide Bandwidth Low Noise, Low Drift Amplifier

• Power BW: fmax = Sr / (2p Vp) >= 240 kHz

• Parasitic input capacitance (C1 approx. 3 pF for LF155, LF156, and LF157 plus any additional layout capacitance) interacts with feedback elements and creates undesirable high frequency pole. To compensate add C2 such that: R2C2 is greater than or equal to R1C1.

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Two Wire To Four Wire Audio Converter

This converter circuit maintains 40 dB of isolation between the input and output halves of a four-wire line while permitting a two-wire line to be connected. A balancing potentiometer, Rg, adjusts the gain of IC2 to null the feed-through from the input to the output. The adjustments is done on the workbench just after assembly by inserting a 1 kHz tone into the four-wire input and setting Rg for minimum output signal. An 82 ohm dummy-load resistor is placed across the two wire terminals.

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Transducer Amplifier

This circuit is high-input impedance ac amplifier for a piezoelectric transducer. Input resistance is 880 M, and a gain of 10 is obtained.

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Summing Amplifier With Low Input Current

(1) Power Bandwidth: 250 kHz
Small Signal Bandwidth: 3.5 Mhz
Slew Rate: 10 V/us
(2) C5 = 6 X 10-8 / R1
(3) In addition to increasing speed, the LM101A raises high and low frequency gain, increases output drive capability and eliminates thermal feedback.

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Stereo Amplifier With Gain Control

Excellent tracking of typical 0.3 dB is easy to achieve. With the potentiometer, Rp, the offset can be adjusted. For ac-coupled amplifiers, the potentiometer may be replaced with two 5.1 k ohm resistors.

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Stable Unity Gain Buffer With Good Speed And High Input Impedance

Q1 and Q2 constitute a simple, high speed FET input buffer. Q1 functions as a source follower, with the Q2 current source load setting the drain-source channel current. Normally, this open loop configuration would be quite drifty because there is no dc feedback. The LTC1052 contributes this function to stabilize the circuit by comparing the filtered circuit output to a similarly filtered version of the input signal. The amplified difference between these signals is used to set Q2's bias and hence Q1's channel current. This forces Q1's Vgs to whatever voltage is required to match the circuit's input and output potentials. The 2000 pF capacitor at A1 provides stable loop compensation. The RC network in A1's output prevent it from seeing high speed edges coupled through Q2's collector-base junction. A2's output is also fed back to the shield around Q1's gate lead, bootstrapping the circuit's effective input capacitance down to less
than 1 pF. For very fast requirements, the alternate discrete component buffer shown will be useful. Although its output is current limited at 75 mA, the GHz range transistors employed provided exceptionally wide bandwidth, fast slewing and very little delay.

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Speaker Amplifier For Hand Held Transceivers

The LM383 is an audio-power amplifier that is capable of producing up to 8 W of audio output. R1 is essentially a load resistor for the hand-held transceiver's audio output. R2 can be composed of two fixed resistors in a 10:1 divider arrangement, but using a potentiometer makes it easy to set the amplifier's maximum gain. When powered from a vehicle's electrical system, the amplifier's +12V power source requires filter L1 to eliminate alternator whine. The LM383 can be mounted directly on the heatsink because the mounting tab is at ground potential.

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Pulse Width Proportional Controller Circuit

The quad operational amplifier circuit yields full 0 to 100 percent pulse-width control. The controller uses an LM3900 that requires only a single supply voltage of 4 to 30 V. The pulse-repetition rate is set by a 1 kHz oscillator that incorporates amplifier A1. The oscillator feeds ramp generator A2, which generates a linear ramp voltage for each oscillator pulse. The ramp signal feeds the inverting input of comparator A3; the speed-control voltage feeds the noninverting input. Thus, the output of the comparator is a 1 kHz pulse train, the pulse width of which changes linearly with the control voltage. The control voltage can be provided by an adjustable potentiometer or by an external source of feedback information such as a motor-speed sensing circuit. Depending on the control-voltage setting, the pulse duration can be set at any value form zero (for zero average dc voltage applied to the motor) to the full pulse-repetition period (for applied motor voltage equal to dc power-supply voltage). An amplifier stage (A4) with a gain of 10 acts as a pulse-squaring circuit. A TIP-31 medium-power transistor is driven by A4 and serves as a separate power-amplifier stage.

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Power Booster

Power booster is capable of driving moderate loads. The circuit as shown uses a NE5535 device. Other amplifiers may be substituted only if R1 values are changed because of the Icc current required by the amplifier. R1 should be calculated from the following expression: R1 = 600 mW/Icc.

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Mini Stereo

This circuit is built around two chips: the MC1458 dual op amp, configured as a preamplifier, and the LM378 dual 4-watt amplifier. The gain of the preamp is given by R3/R1 for one side and R4/R2 for the other side, which is about 100. That gain can be varied by increasing the ratios. The left and right channel inputs are applied to pin 2 and 6. The left and right outputs of U1 at pins 7 and 2 are coupled through C5/R10 and C3/R6, respectively, to U2 to drive the two 8-W loudspeakers.

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Micro Sized Amplifier

Sound detected by electret microphone MIC1 is fed to IC1's input through resistor R2, and capacitors C1 and C2. Resistors R2 and R5 determine the overall stage gain, while C2 partially determines the amplifier's frequency response. To ensure proper operation, use a single-ended power supply. R3 and R4 simulate a null condition equal to half the power supply's voltage at IC1's noninverting input. The output of IC1 is transferred to emitter-follower amplifier Q1 via volume control R6. The high-Z-in/low-Z-out characteristic of the emitter-follower matches the moderately high-impedance output of IC1 to a low-impedance headphone load.

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Low Distortion Audio Limiter

The level at which the audio limiter comes into action can be set with the LIMIT LEVEL trimmer potentiometer. When that level is exceeded, the output from the LIMITER-DETECTOR half of the op-amp (used as a comparator) turns the LED which causes the resistance of the photoresistor to decrease rapidly.
That is turn causes the gain of the LIMITER half of the op-amp to decrease. When the signal drops below the desired limiting level, the LED turns off, the resistance of the photoresistor increases, and the gain of the LIMITER op-amp returns to its normal level-that set by the combination of resistors R1 and R2.
A dual-polarity power supply (+/- 12 volts is desirable) is needed for the op-amp.

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Logarithmic Amplifier 3

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Logarithmic Amplifier 2

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Logarithmic Amplifier

Unusual frequency compensation gives this logarithmic converter a 100 us time constant from 1 mA down to 100 uA, increasing from 200 us to 200 ms from 10 nA to 10 pA. Optional bias current compensation can give 10 pA resolution from -55 degree C to 100 degree C.

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Line Operated Amplifier

T1 isolates the unit from the line, and has a 24-V, center-tapped secondary. The output of the transformer is rectified by diodes D1 and D2 and filtered by capacitor C3 to provide 15 to 18 Vdc. The LM383 has built-in protection against speaker shorts.

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JFET Amp With Current Source Biasing

A current source (MPF102) in the source lead of bipolar transistor 2N3906 permits accurate control of drain current.

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High Input Impedance, High Output Current Voltage Follower

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General Purpose Preamplifier

Suitable for general audio use, the preamp circuit uses a feedback pair. Current gain is set by the ratio of (R4+R6)/R4.

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