NPN PNP BJT Push Pull Emitter Follower Circuit Fragment
Push-Pull Emitter follower circuit fragments take the voltage of a high impedance (can’t provide much current) alternating signal and outputs the same voltage (minus base-emitter voltage drop of bipolar junction transistors called crossover distortion), while amplifying the amount of current. This is a form of power amplification.
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Video below: 2N3904 NPN and 2N3906 PNP BJTs Push Pull Emitter Follower Trimpot Controlled LT1026 Rail Splitter Click here to watch directly on YouTube!
I try to keep my electronics learning as low priced as possible while making maximum use of what I have bought and other people have commonly available. Therefore this demonstration circuit will use the common 5V breadboard breadboard power supply to power a LT1026 rail splitter voltage doubler and inverter to provide a split power voltage source of about +6.5V and -7.5V (under load) to the OV ground reference voltage.
- 5V breadboard power supply – Powers LT1026 integrated circuit which powers the rest of the circuit
- LT1026 wired as voltage doubler and inverter – Takes a single power supply (5V in this case), uses external capacitors to step up and invert the voltage to provide a larger split power supply (about +6.5V to -7.5V in this circuit). Datasheet says 15mA max current might be available for both positive and negative rails. Powers the rest of the circuit. https://youtu.be/oD-WVflamgU LT1026 Integrated Circuit IC Voltage Doubler and Inverter using switched capacitor charge pumps video that I uploaded to YouTube. I got this component from Digikey.com for $4.20 plus shipping.
- 10K trimpot wired as a voltage divider – Connected to both rails, the output pin provides a voltage (signal) at a value anywhere between the rails (approx. +6.5V to -7.5V). However, due to it’s relatively high impedance (resistance) it can’t provide much current at a given voltage without a major voltage drop.
- 2N3904 NPN BJT and 2N3906 PNP BJT wired as Push-Pull Emitter Follower – Powered directly by the rails, they sense the voltage, and apply the same voltage (minus 0.6V base to emitter drop) directly from the power supply (LT1026) to the load which leads to ground (0V).
- Load: A red and a blue LED wired as a polarity indicator (biased in opposite directions) and sharing the same current limiting resistor (1KΩ) – Once there is enough voltage to light the forward biased LED (about 1.6V plus 0.6V B-E drop), you can tell the voltage direction by which LED lights up, and get a sense of the voltage by how bright the LED is. Current calculated by voltage divided by resistance is based on voltage (after drops) divided by the current liming resistor (in this case 1kΩ). Each 1V above the drops will provide 1mA of current due to the 1,000Ω resistor. More current = brighter LED. LEDs should pass no more than 20mA of current. LT1026 is not expected to provide that much current (datasheet say 15mA max possible at both outputs), so 1K should keep the current to well below 10mA since we have less than 10V positive and negative available.
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