Diamond buffer

of push-pull operation The diamond buffer or diamond follower is a four-transistor, two-stage, push-pull, translinear emitter follower, or less commonly source follower, in which the input transistors are folded, or placed upside-down with respect to the output transistors. Like any unity buffer, the diamond buffer does not alter the phase and magnitude of input voltage signal; its primary purpose is to interface a high-impedance voltage source with a low-impedance, high-current load. Unlike the more common compound emitter follower (a "double" in audio engineering terms), where each input transistor drives the output transistor of the same polarity, each input transistor of a diamond buffer drives the output transistor of the opposite polarity. When the transistors operate in close thermal contact, the input transistors stabilize the idle current of the output pair, eliminating the need for a bias spreader. The diamond buffer is used primarily in the input and output stages of high-speed current-feedback operational amplifiers. The circuit is also the essential building block of bipolar current conveyors, and has seen limited use in line-level audio preamplifiers and in the output stages of audio power amplifiers.

of push-pull operation The diamond buffer or diamond follower is a four-transistor, two-stage, push-pull, translinear emitter follower, or less commonly source follower, in which the input transistors are folded, or placed upside-down with respect to the output transistors. Like any unity buffer, the diamond buffer does not alter the phase and magnitude of input voltage signal; its primary purpose is to interface a high-impedance voltage source with a low-impedance, high-current load. Unlike the more common compound emitter follower (a "double" in audio engineering terms), where each input transistor drives the output transistor of the same polarity, each input transistor of a diamond buffer drives the output transistor of the opposite polarity. When the transistors operate in close thermal contact, the input transistors stabilize the idle current of the output pair, eliminating the need for a bias spreader. The diamond buffer is used primarily in the input and output stages of high-speed current-feedback operational amplifiers. The circuit is also the essential building block of bipolar current conveyors, and has seen limited use in line-level audio preamplifiers and in the output stages of audio power amplifiers. Maximum output current into a low-impedance load is thus proportional to the idle current of the first stage: I out.max.sourced =I e1 /β T2 or I out.max.sunk =I e2 /β T4 . At either of these points (which are usually asymmetrical) the base of the respective output transistor intercepts all current available from the source, and the input transistor shuts down, causing abrupt clipping. Maximum output voltage depends on these current-handling limits in conjunction with the load impedance, and on the configuration of constant current sources. Simple resistive current sources, in particular, severely limit maximum currents and voltages. In discrete-transistor circuits, which allow the use of large-value capacitors, both can be improved by bootstrapping the current-setting resistors. Bootstrapping both emitter and collector potentials of T1 and T2 has the added effect of eliminating the Early effect and thus improving linearity. The bootstrapped diamond follower remains fully functional at DC, although the benefits of capacitive bootstrapping show up only at AC signals. The slew rate (SR) of a simple diamond buffer is limited by I e1 at SR=I e1 /C int1 , where the internal capacitance C int1 is the total capacitance "seen" by the current source I e1 at the common node of the base of T2 and the emitter of T1 (or, in case of I e2 , at the common node of the base of T4 and the emitter of T3). For example, a 0.5 mA current source loaded into 10 pF node capacitance has SR of 50 V/μs. Negative and positive slew rates may be markedly asymmetrical. The power bandwidth for peak output voltage V p is limited by the least of these two slew rates at F max =SR/(2πV p ). If the slew rate of the input signal exceeds the SR of the diamond buffer, the circuit may experience thermal runaway - a potentially destructive scenario when both T2 and T4 are conducting. The frequency that triggers thermal runaway for a given peak input voltage V p is defined by the same formula as the power bandwidth, F max =SR/(2πV p ).

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