You lose almost half the range at unity gain, but at higher amplification ratios, the loss is comparatively small. The practical rule to remember is that in this amplifier circuit, Vout can’t drop below Vsignal - Vth. But of course, a total of 12 V is not attainable. If the input signal is 8 V and the supply voltage is 10 V, you might be hoping a 6 V drop across R2 (8 V - Vth), and then a similar drop across R1 that pulls the output -6 V from the positive rail. To understand the consequences, let’s go back once again to the unity gain configuration (R1 = R2). The most significant issue with this circuit - and one glanced over by most online tutorials - is that the sum of voltage drops across the two resistors in series obviously can’t exceed the supply voltage. That minor inconvenience aside, the circuit is a solid amplifier with a well-defined and easily-adjusted gain. A weak input signal with a 400 mV amplitude (yellow) is producing a solid 4 V swing on the output (blue) - a nearly-perfect 10x boost:Īn experiment with a rudimentary transistor amplifier.īecause R1 is dangling from the positive supply rail, the output is inverted: that is, low input voltages produce high output voltages (and vice versa). The current through the circuit would generally remain unchanged and from V = IR, we know that if the current is constant and the resistance increases, we should see a proportionately larger voltage drop across R1.Īnd indeed, we can observe this R1/R2 amplification behavior in the following oscilloscope trace produced for R1 = 100 kΩ and R2 = 10 kΩ. This may seem uninteresting, but consider what happens if R1 is larger than R2. But because the same current is necessarily also flowing through the newly-added R1, the extra resistor will develop a voltage drop across its terminals that is identical to that seen across R2. The fundamental mechanism of the circuit remains the same: the transistor will admit current sufficient to create a self-limiting feedback voltage across R2. Conditioning signals for amplificationįor a moment, let’s assume that R1 = R2. Unlike in a voltage amplifier, the input signal and its amplified version might look the same on an oscilloscope screen. A power amplifier is a device that takes this high-impedance signal and then cranks out a low-impedance copy through a beefy output stage that can supply larger currents with ease. To get 5 V, we’d need to pump out 625 mA.Ī source that can’t supply appreciable currents is said to have high impedance. A single pin on an MCU can at best deliver around 40 mA if it’s driving an 8 Ω speaker, the voltage across the speaker’s terminals will not exceed 320 mV (40 mA * 8 Ω), no matter what the chip is hoping to achieve. The signal’s sudden attenuation is a straightforward consequence of Ohm’s law: V = IR. You might have come across signals that look good when measured in an open circuit, but that drop precipitously when connected to a load a classic example is a microcontroller output pin interfaced directly to a motor or a speaker. The job of a voltage amplifier is to multiply these small readings by some fixed value (“gain”), thus producing an output signal suitable for general use. An example might be the output of an electret microphone, a photodiode, or a temperature sensor - all firmly in the millivolt range. Voltage levels are the primary way of conveying information in electronic circuits, and every now and then, you end up with a signal amplitude that’s too microscopic for other devices to discern. There are two types of amplification circuits you are most likely to encounter: voltage and power amplifiers.
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