Photodiode amplifiers : OP AMP Solutions Graeme, Jerald G.

Graeme, Jerald G.
ISBN: 0-07-024247-X
Год: 1995
Издательство: McGraw-Hill
Город: New-York
Количество страниц: 253
Язык: Английский
Формат: DJVU / RAR

Формат файла: RAR

Photodiodes transform a basic physical indicator, light, into the electrical form commonly used to monitor physical conditions. There, semiconductor junctions convert the photon energy of light into an electrical signal by releasing and accelerating current-conducting carriers within the semiconductor. Specializing the junction design for the photodiode role improves its spectral response and efficiency with PIN and avalanche alternatives. Also, multiple-element and lateral photodiodes provide position-sensing through the relative magnitudes of multiple output currents. By itself, the photodiode can produce a voltage output as required for most electronic instrumentation. However, that operating mode produces a highly nonlinear response and a very restricted bandwidth. Instead, accepting the diode output as a current and performing a current-to-voltage conversion dramatically improves performance. This isolates the photodiode from the signal voltage and makes the op amp current-to-voltage converter the basic photodiode amplifier.
While simple in structure, this circuit still displays surprising multidimensional constraints in photodiode applications. There, the small output levels produced by the photodiode encourage large-area diodes and high-value conversion resistances. However, this combination compromises offset, bandwidth, stability, and noise. These compromises require careful design choices and suggest variations on the basic photodiode amplifier. In the chapters that follow, design equations guide the circuit's component selection, and circuit derivatives optimize the various performance characteristics. In the first circuit
variation, replacing the current-to-voltage conversion resistance with a tee network reduces offset, and a later analysis guides the tee design to avoid the customary noise increase of this replacement.
Next, the high resistance of the conversion resistor and the capacitance of the photodiode combine to produce three separate frequency-response limits. Each potentially sets the bandwidth of the current-to-voltage converter when operated as a photodiode amplifier. These limits result from parasitic capacitance, the limited bandwidth of the op amp, and phase compensation requirements. In the first case, parasitic capacitance bypass of the large conversion resistance rolls off the circuit's response. In other cases, the op amp bandwidth and phase compensation compete to control bandwidth. As a photodiode amplifier, the current-to-voltage converter often produces a two-pole response, resulting from an input circuit that appears like a parallel L-C circuit. This characteristic jeopardizes stability and requires the phase compensation that introduces the third response limit. Feedback analysis develops design equations for this compensation in a compromise that equates the bandwidth limits imposed by the op amp and this compensation.
Reducing the effects of the photodiode capacitance greatly improves bandwidth. Any signal voltage developed across the diode reacts with this capacitance, shunting the diode's output current at higher frequencies. Three circuit methods greatly ease this restriction through signal isolation, photodiode bias, and photodiode bootstrap. Signal isolation removes the signal voltage from the photodiode through the current-to-voltage converter of the basic photodiode amplifier. For even greater bandwidth, biasing or bootstrapping the photodiode further reduces the capacitance effect. Reverse biasing the diode reduces its capacitance but requires additional circuit complexity to minimize the accompanying degradation of offset and noise performance. Bootstrap also isolates the photodiode from signal swing, and combining this with the current-to-voltage converter provides a double degree of isolation.
The photodiode amplifier's resistance, capacitance combination also degrades noise performance, and for higher-gain applications, this noise often controls the amplifier's overall accuracy. As a photodiode amplifier, the current-to-voltage converter typically exhibits a complex noise behavior in which the op amp's input noise voltage receives an unexpected high-frequency gain. Here, breaking the noise analysis into a series of frequency regions restores simplicity. This regional analysis also permits comparison of relative noise effects to identify circuit changes that optimize performance. Frequently, this analysis reveals that the unexpected high-frequency gain or noise gain peaking dominates noise performance. Then, the circuit amplifies the noise voltage of the op amp's input with a noise bandwidth that exceeds the signal bandwidth. Modifications to the basic photodiode amplifier provide filtering to reduce or remove this noise disadvantage. In each case, noise analysis equations guide the circuit's component selection to optimize the circuit's bandwidth-versus-noise compromise.
Bandwidth and noise also compete when maximizing the photodiode amplifier's gain. For the basic photodiode amplifier, just making the feedback resistance large produces high gain. However, the very high resistances often required reduce bandwidth and increase noise. For hose cases, alternative methods optimize this compromise. Increased bandwidth results from supplemental gain provided by several circuit alternatives. The most straightforward implementation simply adds a voltage amplifier following the conventional current-to-voltage converter. A second reduces the circuit complexity by making one op amp serve both the current-to-voltage and voltage gain functions. Current output along with increased bandwidth result from replacing the added voltage gain with current gain. In each case, design equations tfuide component selection for the optimum bandwidth-versus-noise condition.
Noise on the power-supply lines also couples into the signal path of the photodiode amplifier. The finite power-supply rejection ratio of the op amp code couples a portion of this noise to the op amp inputs, and the circuit amplifies this added signal along with the op amp's normal input noise. There, the characteristic noise gain peaking of the photodiode amplifier makes supply noise reduction especially important. In addition, this gain peaking makes the photodiode amplifier more vulnerable to oscillation through a parasitic feedback loop formed through the power-supply lines. Fortunately, capacitive bypass of the power-supply lines greatly attenuates this noise coupling and ensures frequency stability as well. However, to be successful, the bypass selection requires close attention to the frequency-dependent impedances of both the supply lines and the bypass capacitors. Together, they produce multiple resonances in the net supply-line impedance that increase the gain of the parasitic feedback loop. Once again, design equations guide component selection for this bypass function.
Diminishing returns eventually limit the noise reduction achieved through measures focused upon the photodiode amplifier itself. External noise sources impose a background noise floor that requires attention to the amplifier's environment rather than the amplifier. This background noise typically results from the parasitic noise coupling of external electrostatic and magnetic sources. Here again, the noise gain peaking of the photodiode amplifier accentuates the effects of these noise sources. Limiting these effects requires attention to amplifier location, shielding, the circuit structure, and the physical arrangement of circuit components. There, differential amplifier structures and loop-minimizing component arrangements reduce the effects of these external noise sources. Converting the photodiode amplifier to a differential-input form activates the common-mode rejection of the op amp for rejection of both electrostatic and magnetic coupling effects.
In a final set of circuit alternatives, photodiodes provide position-sensing information through the diodes' photo responses. However, several variables potentially degrade the sensing accuracy by introducing offset and gain errors in the light-to-voltage conversion process. Background light introduces a first variable but only adds an offset to the diode output. Differential measurement with two photodiodes removes this offset through the subtraction of two output signals. Variations in the light source intensity and in the photodiode's responsivity introduce additional variables, and these affect the gain of the position measurement. Simple differential monitoring does not remove these effects. However, signal normalization resolves this by dividing two diodes' difference signal by their mean signal. An analog divider extracts this normalized signal with the greatest accuracy, but analog multiplier replacements for the divider simplify the circuitry. Alternately, linear photodiode arrays remove the need for normalization by providing a digital indication of position.
I wish to thank the many photodiode users with whom I have discussed applications requirements over the years. Their shared knowledge and catalytic inquiries prompted the investigations that led to this book. My thanks to EDN and Electronic Design magazines for publishing the articles that initiated many of these discussions. Finally, I wish to thank my wife, Lola, for her accurate and attractive rendering of the illustrations and for the rewarding feeling of mutual involvement in preparing this book.