Mixer Protection: Preamp faults

A mixer can be damaged when one of the power supplies of the mixer preamp is disconnected or goes to zero. When a non inverting opamp is used for the mixer preamp the tail current of its input differential pair may then flow through the mixer diodes. With a bipolar opamp this current may be 1mA or less, with a low noise JFET opamp this current can be much larger. All low noise bipolar opamps include inverse parallel diodes connected between its inverting and non inverting inputs so if the output saturates near either rail then only the feedback resistor and the opamp output stage current limit circuit limit the current that may flow into (or out of) the mixer diodes. This current can be as high as a few tens of mA when a low value feedback resistor is used. Increasing the value of the feedback resistor will reduce the fault current at the expense of increased preamp noise, at high gains the gain determining resistors can attenuate the maximum voltage developed at the inverting input to a value that limits the protection diode current to a few microamps or less.

Minimising mixer current due to preamp faults

A JFET input preamp can be used to limit the fault current when a non inverting opamp output saturates near the rails as there are usually no inverse parallel input diodes connected between the its inputs, however JFET opamps tend to be noisier than bipolar opamps and the input stage tail current may be somewhat larger.

Increase the opamp gain and use higher value feedback resistors, the output can always be attenuated if the required gain is lower provided the opamp doesn't saturate in normal operation. This allows a relatively high value feedback resistor to be used without increasing the equivalent input noise as much as using the same value feedback resistor with a lower gain.

Use an inverting opamp for the preamp. In this case inverse parallel diodes connected between the inverting input and ground are required to limit the current that flows through the input resistor and mixer when the opamp output saturates near one of its supply rails. The combination of the inverting input shunt diodes clamping the inverting node to ground and the input resistor can limit the fault current flowing in the mixer to 1mA or less. However the input resistor loads the mixer and increases the preamp equivalent input noise. The input noise degradation is minimised by using a high preamp gain (limited by opamp output saturation and mixer output) and attenuating the preamp output to achieve the required gain if the resultant preamp gain is too high. When a high output mixer is used the resultant preamp noise may not be significant compared to the mixer output noise. The relatively low input resistance may also be acceptable given the protection this configuration inherently offers to the mixer in the event of a preamp fault.

When a high gain (100x or more) is used then it is easy to ensure that the mixer fault current is very low particularly if a single ended discrete JFET input stage is used. When the mixer output exceeds the preamp linear range dc current from the mixer may flow into the input device gate, however this is limited by the mixer source resistance and is only slightly higher than if a 50 ohm mixer load were used. This can occur if the mixer is operated as a phase detector and the mixer inputs are not in phase quadrature. The gate current may be reduced if a pair of inverse parallel(Schottky) diodes is connected from the gate to ground.


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