1,870
edits
Changes
Jump to navigation
Jump to search
Line 3:
Line 3: + + + + + Line 10:
Line 15: − +
Pulser amplitude feedback amplifier (view source)
Revision as of 16:19, 20 June 2014
, 16:19, 20 June 2014→Circuit diagram and layout
===Circuit diagram and layout===
===Circuit diagram and layout===
The final design is shown in the figures below. It went through considerable evolution during the prototyping phase because we had trouble with high-frequency oscillations of the op-amp. The phase shift between the inputs and the output goes through 180 degrees around 1GHz, but the exact frequency where that occurs turns out to depend a lot on how the output is loaded. The 180 degree point is where instability occurs because at that point feedback to the inverting pin essentially becomes positive feedback. With the following measures, we were eventually able to suppress the oscillations.
# Combine multiple strips around the op-amp region on the circuit board to play the role of a low-inductance ground plane.
# Put a pair of bypass capacitors (10uF, 47nF) between the power pins of the op-amp and the low-inductance ground plane. Locate the 47nF capacitor as close as possible to the power pin on the chip.
# Limit the current on the output pin by putting a 47 Ohm resistor in series with all loads, even the feedback path which might have a ~1 pF of capacitance to ground.
The last step proved to be crucial. The manufacturer's data sheet warns about instability if the output pin is not protected by a resistor, and even gives a table of recommended resistance values depending on the capacitance of the output net to ground. At first I resisted this because it means giving up output amplitude, but it turned out to be the key step needed to achieve stability.
[[image:laserDiode_pd_amp(1).png|600px|center]]
[[image:laserDiode_pd_amp(1).png|600px|center]]
<blockquote>
<blockquote>
[[image:laserDiode_pd_amp(2).png|600px|center]]
[[image:laserDiode_pd_amp(2).png|600px|center]]
<blockquote>
<blockquote>
Figure 2. Layout of the pulser amplitude feedback amplifier circuit board, showing external connections to the 9V power supply, the laser diode package, and the output signal which generates the amplitude feedback pulses. The layout is designed for a standard prototype PCB with traces running in one direction across the board and holes on a 1/10 inch grid. The 9V battery is mounted on the back side of the board. External wires to the laser diode are labeled as 2 and 3. Capacitors are colored purple, resistors are light brown, and wired connections are gray or green.
Figure 2. Layout of the pulser amplitude feedback amplifier circuit board, showing external connections to the 9V power supply, the laser diode package, and the output signal which generates the amplitude feedback pulses. The layout is designed for a standard prototype PCB with traces running in one direction across the board and holes on a 1/10 inch grid. The 9V battery is mounted on the back side of the board. External wires to the laser diode are labeled as 2 and 3. Capacitors are colored purple, resistors are light brown, and wired connections are gray or green. Filled circles represent wired connections on the back side of the board, with the fill color indicating the matched pairs.
</blockquote>
</blockquote>