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Friday, March 26, 2010

Simple Isolating A Capacitive Load Circuit

This is a simple design circuit for capacitive load. This circuit is based on LF411 or LF441 op amps IC. This is the figure of the circuit;

Both the LF411 and LF441 families have a lot of trouble driving more than about 200 pF without oscillating. Standard techniques to get around this problem are to add a small resistor (about 50X) in series with the output or to use one of the LF356 family (the 356 itself being the most popular for this purpose). An explanation of why the 356 output stage is so unusually strong is provided in the second reference listed. When extra output filtering is desired, a series RC damper will often be more effective than just a large filter capacitor.

[Circuit source: National Semiconductor Notes].

Thursday, March 25, 2010

Simple Ultra High Input Impedance AC Unity Gain Amplifier

This is a design simple ultra high input impedance AC unity gain amplifier circuit. This circuit is used to get unity gain from ultra high input impedance AC. This is the figure of the circuit.

Because we can’t reduce input capacitance of this circuit, so we use the 2N4416 which has low capacitance. It is operated as a source follower with bootstrapped gate bias resistor and resistor.

[Circuit schematic source: National Semiconductor Application Note]

Simple Operational Amplifier (Op-Amp) Oscillator Circuit

This is Operational an Amplifier (Op-Amp) Oscillator circuit. This circuit has some advantages, they are this circuit can be operated at low frequencies with relatively small capacitors, it has a completely-symmetrical output waveform along with a buffered output and it will always self start and cannot hang up because there is less dc positive feedback than negative feedback. Returning R2 to some voltage other than ground will change the symmetry. This is the figure of the circuit.

The usual frequency compensation components (capacitor and resistor across pin 1 and 8) are not needed in this circuit because the operational amplifier is used in open-loop configuration. It will only slow the performance if we use frequency compensation components. Moreover, the use of the circuit is restricted to frequencies below about 2 KHz by the speed limitations of LM101, even without a 30 pF compensation capacitor.

The C1 acts as timing capacitor that is produces several times constant which is used to permit large voltage swings on the input due to the LM101’s large input voltage range. To keep from exceeding these ratings, the R2 should be reduced and increase the C1. The smaller polarized capacitor is still used, although C1 requires the large values, by returning them to positive supply voltage instead of ground.

[Circuit schematic source: National Semiconductor Application Note]

Simple Lie Detector Circuit

To know someone is lying or not we can use lie detector. This is a simple lie detector that can be made in minutes. It works to detect someone who telling lie but it is not as sophisticated as the ones the professionals use. This is the figure of the circuit.

The skin resistance will go down when someone telling lie and this circuit works by measuring it. We can use electrode pads, alligator clips, or just wires and tape as the electrodes. We have to adjust R2 to position the meter at the center at the beginning of the interrogation (relax condition), then we can know someone is lying when the meter changes in response to a question.

[Circuit schematic source:]

Simple Heartbeat Sensor (Transducer) Circuit

To build a heart-beat transducer not as difficult as imagined. This is one design for simple heat sensor. This is the figure of the circuit.

This circuit made from an infrared phototransistor and infrared LED. This transducer works with the principle of light reflection, in this case the light is infrared. The skin is used as a reflective surface for infrared light. The density of blood in the skin will affect on the IR reflectivity. The pumping action of heart causes the blood density rises and falls. So that we can calculate the heart rate based on the rise and fall of intensity of infrared that reflected by skin.

Saturday, March 13, 2010

Simple Low Distortion Oscillator Circuit

This is the design circuit for oscillator. This circuit is based on FET as controller. This is the figure of the circuit;

With very low harmonic distortion, 2N4416 JFET is suitable to smoothly oscillate in a circuit. On application where low harmonic content is required for a good mixer circuit, JFET local oscillator is very good.

And here’s the 20 MHz Oscillator Values
· C1 = 700 pF L1 = 1.3 mH
· C2 = 75 pF L2 = 10T 3/8″ DIA 3/4″ LONG
· VDD = 16V ID = 1 mA
· 3RD HARMONIC > -70 dB

[Circuit Schematic Source: National Semiconductor Application Note]

Simple JFET Sample and Hold Circuit

This is JFET sample and hold circuit. In this circuit, the logic voltage is applied simultaneously to the sample and hold JFETs. This is the figure of the circuit;

The errors due to rds (on) of the JFETs can be minimized by matching feedback capacitance and resistance and input impedance. The circuit performance is greatly improved because of matched leakage currents of the FM1109 monolithic dual and the inherent matched rds (on). [Circuit Schematic Source: National Semiconductor Application Note]

Thursday, March 11, 2010

Simple Programmable Voltage Regulator Circuit Using L200 IC

This is a circuit a programmable voltage regulator circuit. With this circuit we can generate a voltage according to our wishes. This circuit uses L200 IC. This is the figure of the circuit.

Potentiometer R2 is used to fix the output voltage Vo. Following equation show value of the output voltage:

Vo=Vref (1+(R2/R1))

We can obtain a wide range of fixed output voltage by substituting the potentiometer with a fixed resistor and choosing suitable values for R2 and R1. [Source: SGS-Thomson Application Note].

Simple Precision Current Sink Circuit

This is a design circuit for simple circuit for precision current sink. This circuit uses two kinds of transistor, 2N2219 bipolar transistor and 2N3069 JFET because both of them have inherently high output impedance. This is the figure of the circuit.

To provide feedback to the LM101 op amp, this circuit uses R1 as a current sensing resistor. The true current sink nature of this circuit is enhanced because the op amp provides a large amount of loop gain for negative feedback.

The output current is given by:
Io = Vin/R1
with Vin > 0 V

The 2N2219 and the 10K resistor may be eliminated, for small current values, if the JFET’s source is connected to R1. [Source: National Semiconductor Application Note].

Saturday, March 6, 2010

Simple Voltage Controlled Variable Gain Amplifier Circuit

This is a simple circuit of voltage controlled variable gain amplifier. This circuit uses 2N4391 that will provide a low Rds(on) less than 30 Ohm. This is the figure of the circuit.

This circuit is a simplest form. This circuit also uses three attenuators, that it will provide optimum dynamic linear range for attenuation. We can obtain the attenuation greater than 100dB at 10MHz by providing proper RF construction techniques.

Simple Differential Analog Switch Circuit

This is a design simple circuit of differential analog switch circuit. This circuit uses base on the FM1208 monolithic dual. This is the figure of the circuit.

This circuit is ideal choice for an accurate multiplexer because RDS (on) for the monolithic dual tracks at better than ± 1% over wide temperature ranges (-25 to +125°) although it is unusual. [Circuit Schematic Source: National Semiconductor Application Note].