Bi-Directional Charge Coupled Powers MOSFET Relay  -  The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line . . . [Hobby Circuit designed by David Johnson P.E.,  06/03/00]

Characteristics of CMOS Logic Inverter Amplifiers  -  This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . [Hobby Circuit designed by David Johnson P.E.,  02/11/02]

Charge Coupled Bi-Directional Power MOSFET Relay  -  The circuit uses an inexpensive C-MOS inverter package and a few small capacitors to drive two power MOS transistors from a 12v to 15v supply. Since the coupling capacitor values used to drive the FETs are small, the leakage current from the power line . . . [Circuit by David A. Johnson P.E.,  06/03/00]

Charge Coupled MOSFET Relay #2  -  This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current.  The differential oscillator supplies voltage to the gates of the two FETs with good isolation will drawing only 1.5ma of current . . . [Circuit by David A. Johnson P.E.,  10/16/05]

Charge Pump Efficiently Divides Supply Voltage   -  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value . . . [Circuit by David Johnson P.E.,  07/08/00]

Circuit Protects from Battery Polarity Reversal  -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was . . . [Circuit by Dave Johnson P.E.,  03/09/02]

CMOS Logic Inverter Amplifier Characteristics  -  This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . [Circuit by Dave Johnson P.E.,  02/11/02]

FET Input High Speed Light Detector  -  This circuit is yet another design that converts current from a PIN photo diode to a voltage.  It has a bandwidth that extends beyond 50MHz . . . [Circuit by Dave Johnson P.E.,  06/17/00]

FETs and Logic Devices Divides Supply Voltage on Charge Pump Efficiently   -  This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/08/00]

Form Push-Pull Driver using N-Ch And P-Ch Transistors   -  This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts. The output current is only limited by the rating of . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/27/00]

High Impedance JFET Buffers  -  I have used these circuits many times.  They are great when you need a low gain AC signal amplifier with a very high input impedance.  It is good to beyond 50MHz . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]

High Speed Light Detector from FET Input  -  This circuit is yet another design that converts current from a PIN photo diode to a voltage. It has a bandwidth that extends beyond 50MHz . . . [Hobby Circuit designed by David Johnson P.E.,  06/17/00]

Improve Response Time of a Photo Transistor Circuit  -  This page shows different ways to improve the response time of a photo transistor circuit . . . [Hobby Circuit designed by David Johnson P.E.,  11/01/10]

JFET High Impedance Buffers  -  I have used these circuits many times.  They are great when you need a low gain AC signal amplifier with a very high input impedance.  It is good to beyond 50MHz . . . [Circuit by David A. Johnson P.E.,  07/11/06]

N-Ch & P-Ch Transistor Form Push-Pull Driver  -  This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts. The output current is only limited by the rating of . . . [Circuit by David A. Johnson P.E.,  06/27/00]

NPN/PNP Transistor Tester  -  I designed this circuit about 30 years ago and still use it from time to time. It is powered from a 3v battery. It makes a tone when the transistor tests good. It can be used to determine device type (NPN or PNP) and the base, collector or emitter transist . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/28/00]

Peak to Peak 300v Signal Generator  -  This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/03/00]

Protects from Battery Polarity Reversal  -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was . . . [Hobby Circuit designed by Dave Johnson P.E.,  03/09/02]

Sink to Source Universal DC Converter  -  As illustrated below, many control systems use an open collector transistor to turn something on.Perhaps it is a LED light, a relay or a small motor.However there are othersystems which require a common ground connection instead of a common V+ supply connection . . . [Circuit by David A. Johnson P.E.,  03/27/11]

Solid State Touch Activated Switch  -  It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/03/00]

Temperature Sensor MOSFET  -  Recently I was asked by a company to discover why one of their products failed to operate properly in cold temperatures.  Two n-channel MOSFETs and a few resistors are wired to form a flip/flop latch circuit.  Each time the pushbutton switch is pressed, the circuit changes state and remains in that state until another activation of the switch.  . . . [Circuit by David Johnson P.E.,  12/01/10]

Universal DC Sink to Source Converter  -  As illustrated below, many control systems use an open collector transistor to turn something on.  Perhaps it is a LED light, a relay or a small motor.  However there are other  systems which require a common ground connection instead of a common V+ supply . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/27/11]

Very Low Power Astable Multivibrator  -  This classic circuit draws only 200 nanoamps from a 1.5v supply . . . [Circuit by David A. Johnson P.E.,  06/10/00]