LED DRIVER

Description: An input sine wave applied to the left amplifier base results in the collector output fluctuating above or below 6.6 volts. The amplifier amplifies and inverts the signal, maintaining a roughly centered output, thereby minimizing the chances of clipping. The emitter voltage slightly shifts the collector voltage upward but does not significantly affect the operation. Feedback from the emitter enhances fidelity. In contrast, applying a sine wave to the right amplifier base causes the collector output to rise above or fall below 11 volts. This amplifier also amplifies and inverts the signal; however, the output is poorly centered, leading to a higher likelihood of clipping due to the large collector resistor affecting the circuit's base bias. A negative input signal can easily cutoff the collector signal, as the quiescent collector voltage is 11 volts, requiring only a one-volt increase to saturate. The positive input signal can cause the collector to drop by 11 volts, compared to 5.4 volts for the left amplifier, resulting in a highly asymmetrical output waveform. The clipping effect has been observed when using this amplifier as a microphone preamp.

Switching to the LED driver, replacing one LED for the display introduces problems similar to those previously mentioned. The LED driver was designed to output an average of 80 milliamperes for 12 LEDs, while substituting a single LED, which may require 20 mA depending on the type, leads to overdriving the LED. The driver gain remains unchanged, but the increased load resistance results in poor fidelity and linearity. The absence of a series limiting resistor for the LED circuit can damage the LED.

Regarding input voltage, it is unclear what voltage is being used. If the standard 1 volt from the NBTVA club is employed, the CD volume may be set too high for the amplifier. The PNP input transistor junction requires approximately 0.7 volts to turn on, and the bias network provides this voltage under no-signal conditions. This quiescent voltage turns the LEDs on halfway, with the signal adding to or subtracting from this bias. Testing the circuit with a Spice simulator using generic transistor models reveals significant nonlinearity, predicting saturation at input signals above 0.25 volts peak. This suggests the use of germanium transistors, which turn on at lower voltages compared to silicon devices that require approximately 0.7 volts for proper biasing in class-A operation. For optimal performance, silicon transistors should be utilized in this circuit.

The fidelity concerns may be addressed using simulation software, although the author lacks access to such tools. For a 1-volt input signal, designing an attenuator with the Spice simulator is recommended, or alternatively, adjusting the volume on the input CD player may suffice.

In summary, the circuit analysis indicates that both amplifiers exhibit unique characteristics affecting their performance, with the left amplifier maintaining a centered output and the right amplifier suffering from clipping and asymmetry. The LED driver must be carefully designed to avoid overdriving LEDs, and the input voltage should be managed to ensure proper transistor biasing for optimal operation.With an an input sinewave on the left amplifier base, here`s what happens: The collector output rises above or falls below 6. 6 volts. Of course, the amplifier increases signal level and inverts the signal. Because the output is roughly centered, the chances of clipping are small. (The emitter voltage shifts the collector voltage upward a bit, but not significantly. In return, emitter feedback tends to improve fidelity. ) Right Amplifier Analysis. Now, we apply an input sinewave to the right amplifier base. The collector output rises above or falls below 11 volts. Again, the amplifier increases signal level and inverts the signal. Unfortunately, the output is far off center. The chance of clipping is likely. (The large collector resistor upset the circuit`s base bias. ) A negative input signal easily cuts off the collector signal. Remember that the collector quiescent voltage is 11 volts. The collector only needs to rise by one volt. In the positive direction, this amplifier has a lot of gain. A positive input signal can cause the collector to drop 11 volts. Compare that to 5. 4 volts for the left amplifier. Unfortunately, the right amplifier`s output waveform is terribly assymmetrical. I can vouch for the clipping effect. I demonstrated it. I wired this amplifier to a signal tracer. Then I used the circuit as a microphone preamp. I could hear the output distortion. Now, back to the LED driver. By substituting one LED for my display, you introduce similar problems to those in the example above. I designed the LED driver to output 80 milliamperes average to 12 LEDs. You substituted one LED. This LED probably requires 20 mA. (That depends on what LED type you used. ) The result You overdrive the LED. Why Because you aren`t using the rated load. Driver gain doesn`t change, but load resistance increases. No wonder fidelity and linearity are poor. Your "simulation" guarantees that result. Also, you don`t mention a series limiting resistor for the LED circuit. If you forget the resistor, the LED is toast. Input Voltage. Now, let`s talk about input voltage. I don`t know what voltage you`re using for the input. Did you use the NBTVA club standard of 1 volt Then you`ve probably got the CD volume up too high for this amplifier.

To turn on, the PNP input transistor junction requires about 0. 7 volt. For normal bias without a signal, the bias network gives my transistor 0. 7 volt. (Because this is a PNP transistor, the polarity is really -0. 7 volt. ) This quiescent voltage turns the LEDs on halfway. The signal adds to or subtracts from this quiescent (no-signal) bias. PROBLEM. I tested this circuit on a Spice simulator. I used "generic" transistor models, not the parts you prescribed. I see lots of nonlinearity. With a 1-volt drive signal, the model predicts bad behavior. At anything above 0. 25 volts peak input, the circuit saturates. I don`t know what type of transistors you substituted in your simulation. Since your circuit saturates at 0. 25 volts, I suspect germanium transistors. Germanium devices turn on at 0. 25 to 0. 3 volt or so. This is the difference voltage between the base and emitter. Saturation occurs at some higher voltage. Silicon devices turn on with a difference voltage of 0. 7 volt. This is the proper bias voltage for class-A operation. If you expect the best performance, be sure to use silicon transistors in this circuit. As to the fidelity criticism, you have the fancy software, and maybe that tells you something I don`t see. I can`t speak to that. I don`t have the software. I just built the circuit. If you insist on a 1-volt input signal, here`s a suggestion: Maybe you need to fire up that Spice simulator and design an attenuator.

Whatever floats your boat. I`d just twist the volume knob on the input CD player. ANSWER. If you have a design that you think is superior, then build it. Experimenting is one of the best ways to learn