Improved 3 Transistor Audio Amp (80 milliwatt)

This circuit is similar to the one above but uses positive feedback to
get a little more amplitude to the speaker. I copied it from a small
5 transistor radio that uses a 25 ohm speaker. In the circuit above,
the load resistor for the driver transistor is tied directly to the
+ supply. This has a disadvantage in that as the output moves positive,
the drop across the 470 ohm resistor decreases which reduces the
base current to the top NPN transistor. Thus the output cannot
move all the way to the + supply because there wouldn't be any
voltage across the 470 resistor and no base current to the NPN transistor.


This circuit corrects the problem somewhat and allows a larger
voltage swing and probably more output power, but I don't know
how much without doing a lot of testing. The output still won't
move more than a couple volts using small transistors since the
peak current won't be more than 100mA or so into a 25 ohm load.
But it's an improvement over the other circuit above.




In this circuit, the 1K load resistor is tied to the speaker so
that as the output moves negative, the voltage on the 1K
resistor is reduced, which aids in turning off the top NPN transistor.
When the output moves positive, the charge on the 470uF capacitor
aids in turning on the top NPN transistor.



The original circuit in the radio used a 300 ohm resistor where
the 2 diodes are shown but I changed the resistor to 2 diodes so
the amp would operate on lower voltages with less distortion.
The transistors shown 2n3053 and 2n2905 are just parts I used
for the other circuit above and could be smaller types.
Most any small transistors can be used, but they should be
capable of 100mA or more current. A 2N3904 or 2N3906 are probably
a little small, but would work at low volume.




The 2 diodes generate a fairly constant bias voltage as the battery
drains and reduces crossover distortion. But you should take care
to insure the idle current is around 10 to 20 milliamps with
no signal and the output transistors do not get hot under load.



The circuit should work with a regular 8 ohm speaker, but the
output power may be somewhat less.
To optimize the operation, select a resistor where the 100K is shown
to set the output voltage at 1/2 the supply voltage (4.5 volts).
This resistor might be anything from 50K to 700K depending on
the gain of the transistor used where the 3904 is shown.

Audio from a telephone line can be obtained using a transformer and capacitorto isolate the line from external equipment. A non-polarized capacitor isplaced in series with the transformer line connection to prevent DC currentfrom flowing in the transformer winding which may prevent the line fromreturning to the on-hook state. The capacitor should have a voltage ratingabove the peak ring voltage of 90 volts plus the on-hook voltage of 48 volts,or 138 volts total. This was measured locally and may vary with location,a 400 volt or more rating is recommended. Audio level from the transformer isabout 100 millivolts which can be connected to a high impedance amplifier ortape recorder input. The 3 transistor amplifier shown above can also be used.For overvoltage protection, two diodes are connected across the transformersecondary to limit the audio signal to 700 millivolts peak during the ringingsignal. The diodes can be most any silicon type (1N400X / 1N4148 / 1N914or other). The 620 ohm resistor serves to reduce loading of the line if theoutput is connected to a very low impedance.

LED Photo Sensor

Here's a circuit that takes advantage of the photo-voltaic voltage of an ordinary
LED. The LED voltage is buffered by a junction FET transistor and then applied
to the inverting input of an op-amp with a gain of about 20. This produces a
change of about 5 volts at the output from darkness to bright light. The 100K
potentiometer can be set so that the output is around 7 volts in darkness and
falls to about 2 volts in bright light.

Triangle and Squarewave Generator

Here is a simple triangle/squarewave generator using a common 1458 dual
op-amp that can be used from very low frequencies to about 10 Khz.

The
time interval for one half cycle is about R*C and the outputs will
supply about 10 milliamps of current. Triangle amplitude can be altered
by adjusting the 47K resistor, and waveform offset can be removed by
adding a capacitor in series with the output.

Low Frequency Sinewave Generators

The two circuits below illustrate generating low frequency sinewavesby shifting the phase of the signal through an RC network so thatoscillation occurs where the total phase shift is 360 degrees. The transistorcircuit on the right produces a reasonable sinewave at the collectorof the 3904 which is buffered by the JFET to yield a low impedanceoutput. The circuit gain is critical for low distortion and you may needto adjust the 500 ohm resistor to achieve a stable waveform with minimumdistortion. The transistor circuit is not recommended for practicalapplications due to the critical adjustments needed.

The op-amp based phase shift oscillator is much more stable than thesingle transistor version since the gain can be set higher thanneeded to sustain oscillation and the output is taken from theRC network which filters out most of the harmonic distortion.The sinewave output from the RC network is buffered and the amplituderestored by the second (top) op-amp which has gain of around 28dB. Frequencyis around 600 Hz for RC values shown (7.5K and 0.1uF) and canbe reduced by proportionally increasing the network resistors (7.5K).The 7.5K value at pin 2 of the op-amp controls the oscillator circuit gainand is selected so that the output at pin 1 is slightly clipped at thepositive and negative peaks. The sinewave output at pin 7 is about 5 voltsp-p using a 12 volt supply and appears very clean on a scope since theRC network filters out most all distortion occurring at pin 1.

Touch Activated Light

The circuits below light a 20 watt lamp when the contacts aretouched and the skin resistance is about 2 Megs or less.The circuit on the left uses a power MOSFET which turnson when the voltage between the source and gate is around6 volts. The gate of the MOSFET draws no current so thevoltage on the gate will be half the supply voltage or6 volts when the resistance across the touch contacts isequal to the fixed resistance (2 Megs) between the source and gate.
The circuit on the right uses three bipolar transistors toaccomplish the same result with the touch contact referencedto the negative or ground end of the supply. Since the baseof a bipolar transistor draws current and the current gain isusually less than 200, three transistors are needed to raisethe microamp current level through the touch contacts to a coupleamps needed by the light. For additional current, the lamp could bereplaced with a 12 volt relay and diode across the coil.

AC Line Current Detector

This circuit will detect AC line currents of about 250 mA or more without
making any electrical connections to the line. Current is detected by passing
one of the AC lines through an inductive pickup (L1) made with a 1 inch
diameter U-bolt wound with 800 turns of #30 - #35 magnet wire. The pickup
could be made from other iron type rings or transformer cores that allows
enough space to pass one of the AC lines through the center. Only one of the
current carrying lines, either the line or the neutral should be put through
the center of the pickup to avoid the fields cancelling. I tested the circuit
using a 2 wire extension cord which I had separated the twin wires a small
distance with an exacto knife to allow the U-bolt to encircle only one wire.


The magnetic pickup (U-bolt) produces about 4 millivolts peak for a AC line
current of 250 mA, or AC load of around 30 watts. The signal from the pickup
is raised about 200 times at the output of the op-amp pin 1 which is then
peak detected by the capacitor and diode connected to pin 1. The second
op-amp is used as a comparator which detects a voltage rise greater than the
diode drop. The minimum signal needed to cause the comparator stage output
to switch positive is around 800 mV peak which corresponds to about a 30 watt
load on the AC line. The output 1458 op-amp will only swing within a couple
volts of ground so a voltage divider (1K/470) is used to reduce the no-signal
voltage to about 0.7 volts. An additional diode is added in series with the
transistor base to ensure it turns off when the op-amp voltage is 2 volts.
You may get a little bit of relay chatter if the AC load is close to the
switching point so a larger load of 50 watts or more is recommended. The
sensitivity could be increased by adding more turns to the pickup.