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1. Sample and Hold Amplifier

2. Oscillator

3. Voltage Controlled Amplifier

4. Exclusive NOR Gate

5. Thermistor Amplifier

6. One-Shot Timer


These ideas are only springboards for your circuit creativity. And to boot, there are no single "right" answers. Let the design have several starts and stops, a few zig-zag paths, its okay, just have fun and get in the game.

So take off on an idea that strikes your creative spirit. Play with some circuit elements to get your idea going. Think of the circuit components as Lego building blocks ( Lego from the Danish word meaning to "play well".) As a suggestion, take a particular angle on the design: all opamps, only transistors, just SPICE sources with table or equation definitions, or any other approach you might like.

Let us know about any interesting circuits that pop out of your design adventures. Share any challenges and solutions you've encountered. Send us a netlist and a brief description. (Circuit should be small enough to run on a demo version.)


 1  -  SAMPLE AND HOLD AMPLIFIER                                                

Create a sample and hold circuit using a SPICE Switch or JFET followed by a capacitor. You can also buffer the output with a high impedance amplifier.

As an example, sample a 1 kHz sinewave with a 10 kHz sampling pulse that's 1us wide. Use the SIN and PULSE definitions for these waveforms.


  2  -  OSCILLATOR                                                                                     

Create an oscillator based on a bandpass filter followed by a comparator stage. The comparator output drives the bandpass filter.

Bandpass filter ideas: LCR filter, opamp filter with R/Cs only.

Camparator ideas: transistor, opamp, E source with table definition.


  3  -  VOLTAGE CONTROLLED GAIN AMPLIFIER                         

Design an amplifier whose gain is controlled by an external voltage. The parameters could be anything. As an example: vary the control voltage from 0 to 5V to change the gain from 1 to 10 V/V.

Amplifier ideas: An opamp where one of the gain resistors is made up of a JFET or a SPICE Switch device (voltage controlled resistance). Or, a transistor differential pair whose emitter current is controlled by an external voltage.


  4  -  EXCLUSIVE NOR GATE                                                                       

Create an Exclusive NOR gate (output is HI when inputs A AND B are both HI or A AND B are both LO.)  As your building blocks, combine some of the Basic Digital Gates found in the circuit collection.


  5  -  THERMISTOR AMPLIFIER                                                                    

Suppose you need to design an amplifier for a thermistor sensor that outputs +0.1 V / deg C with the output equal to +2.5 V at +25 deg C.

Amplifier ideas: A great place to start is the Linearized Thermistor Circuit. Follow this by an opamp whose gain scales the output to +0.1 V / deg. Use the same or another opamp to shift the output to +2.5 V at +25 deg C.

  6  -  ONE SHOT TIMER                                                                                         

Change the 555-Timer from oscillator mode to one-shot mode. Use a repeating PULSE waveform as the trigger.




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