circuit analysisdiodesRF
I've been told that these are disconnect diodes. However, I am unsure as to what purpose they serve. They are used on the line that sends a signal to a piezoelectric, the return signal from the piezoelectric returns on the same path but has a RX/TX switch that doesn't allow the initial signal in.
simulate this circuit – Schematic created using CircuitLab
If we redraw your circuit without the diodes, we have this:
simulate this circuit – Schematic created using CircuitLab
Now imagine you drive Column-1 High and Row-1 Low (and Column-2 and Row-2 are Hi-Z) because you're trying to activate L1 in one direction.
You'll find that you'll also have current flowing through L2, L4 & L3 since the 3 of them in series are connected in parallel with L1.
The same circuit, drawn a little differently, but keeping all of the connections the same could look like this:
and now its much easier to see how this would work.
So, the diodes are there to isolate the coils form each other to prevent this situation and allow each coil to be driven independently.
But what is the purpose of the anti-parallel switching diodes, D1-D2? They appear to be in a clipping/limiting configuration, but I cannot figure out why they're needed.
Without further information regarding your schematic, the diodes (D1, D2) aren't needed for the circuit to operate.
When +15 volt input pulse is applied, the MOSFET (M1) will turn on. When +15 pulse is removed (input volts = 0), the M1 remains on (at least as long as the gate charge remains). When -15 is applied, M1 will turn off and remain off.
It is D3 (zener) that is used to block discharge of the gate, so that M1 remains on even when input pulse goes from +15 to zero volts.
Green = Input pulse V1
Red = Voltage at gate of M1
Yellow= current in R1 (10 ohm resistor), effectively gate current
The diodes (D1 and D2) just cause a diode voltage drop on the input pulse. The diodes would also provide some (minor) damping (just like R1 would) if inductance was in the gate circuit and caused ringing.
EDIT : The following is without the anti-parallel diodes (replaced with a wire).
Best Answer
In this application, the forward voltage drop of the diodes means that they have a relatively high impedance for voltages less than about ±0.65V, which effectively disconnects the source impedance of the pulse driver from the transducer when receiving, preventing the driver from "loading down" the high impedance of the sensor and reducing the overall sensitivity.
When transmitting, the drop is negligible.