![]() ![]() The same can be said for their emitter voltage. Since the transistors and collector resistors are identical, this means that when collector voltage drops by a certain amount at, then the collector voltage at increases by the same amount. That means that for every 1mV of signal increase at transistor ‘s base, there is an equal and opposite 1mV of signal decrease at transistor ‘s base. To find the gain here, we need to realize one important characteristic of this circuit. Now we need to express v_ as a function of our input signal For each transistor however, is different: Using the transistor’s equivalent circuit (see here for more details), we know that each collector’s current is: ![]() To find the differential gain, instead of applying the same signal to both inputs, we apply a differential input, as follows:Ī little math is required to find this circuit’s gain. In practical circuits the differential output will always show some minute voltages due the transistors not being perfectly matched, and due to the resistors’ tolerances. Taking the differential output ( ) will yield zero. The output is identical regardless of which output point A or B is used. Each transistor is in a classic common emitter configuration, so gain is. Indeed, both legs of the differential amplifier handle the same current, and the same current will flow through each resistor: Therefore, opening the circuit at point C is of no consequence. ![]() Now, we know that the transistor currents don’t flow from one transistor to the other, but instead go through the common emitter resistor(s). First, replace by an equivalent resistor network consisting of two resistors in parallel, like so: So how do we find the true gain of this circuit? A neat little trick outlined in The Art of Electronics simplifies this process nicely. The reason this is not correct is because the common emitter resistor sees twice the current that either transistor handles. A common mistake would be to assume that the gain of this circuit is simply. At point A, the two identical currents flowing from both transistors flow into the common emitter resistor. Therefore, since the same signal is applied to both transistors, the same current flows through them both. The two transistors are identical, and both collector resistors are identical. Let’s apply an identical signal to both inputs. The differential gain is determined with a differential input signal, while the common-mode gain is determined with the same signal applied at both inputs. When using differential amplifiers, two different gains can be calculated: the differential gain, and the common-mode gain. using both outputs: this is called differential output.using one output: this is called single-ended output. ![]()
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