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A unity gain buffer amplifier may be constructed by applying a full series negative feedback (Fig. 2) to an op-amp simply by connecting its output to its inverting input, and connecting the signal source to the non-inverting input (Fig. 3). Unity gain here implies a voltage gain of one (i.e. 0 dB), but significant current gain is expected. In ...
The net open-loop small-signal voltage gain of the op amp is determined by the product of the current gain h fe of some 4 transistors. In practice, the voltage gain for a typical 741-style op amp is of order 200,000, [16] and the current gain, the ratio of input impedance (~2−6 MΩ) to output impedance (~50 Ω) provides yet more (power) gain.
Date/Time Thumbnail Dimensions User Comment; current: 16:08, 31 August 2012: 720 × 720 (7 KB): Michael9422 {{subst:Upload marker added by en.wp UW}} {{Information |Description = {{en|A simple NPN transistor amplifier circuit diagram with transistor labels.}} |Source = I created a postscript file, and converted it too SVG using the pstoedit program. |Date = ...
A circuit diagram of a buffer amplifier made using an operational amplifier. = = (realistically, the differential input impedance of the op-amp itself, 1 MΩ to 1 TΩ) Date: 26 January 2009: Source: Own work
The current gain is unity, so the same current is delivered to the output load R L, producing by Ohm's law an output voltage v out = v Thév R L / R S, that is, the first form of the voltage gain above. In the second case R S << 1/g m and the Thévenin representation of the source is useful, producing the second form for the gain, typical of ...
The input signal is applied across the ground and the base circuit of the transistor. The output signal appears across ground and the collector of the transistor. Since the emitter is connected to the ground, it is common to signals, input and output. The common-emitter circuit is the most widely used of junction transistor amplifiers.
Thus this circuit finds applications as a voltage buffer. In other words, the circuit has current gain (which depends largely on the h FE of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load.
The peaking of the gain curve is typical of uncompensated or poorly compensated transimpedance amplifiers. The curve labeled A OL is the open-loop response of the amplifier. The feedback factor, plotted as a reciprocal, is labeled 1/β. In Fig. 4 the 1/β curve and A OL form an isosceles triangle with the frequency axis.