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LM2901: Diode of input section

cafain
Mastermind 6835 points

Part Number: LM2901
Other Parts Discussed in Thread: LM324, LM339

Hi, Staff

Please tell me about the input circuit of the LM2901.
Let me question the circuit shown in Figure 6 on page 11 of the LM2901 data sheet (SLCS006U).

1.What is the role of the diode between the base and emitter of the PNP-transistor connected to the input terminals (IN+ and IN-)?
2.What is the role of the diode connected to the Current Regulator?

best regards
cafain

  • 0
    TI__Guru 69956 points

    Hello Cafain,

    Both diodes are there to protect the input stage when the inputs are taken above VCC.

    The diode across the B-E is to protect against reverse base-emitter breakdown. It will clamp the reverse B-E to a diode drop.

    The drawing is greatly simplified, but there is a second current source (signified by the 10uA) feeding the emitter of the PNP transistor. This is done to keep the bias currents stable. The diode is there to "isolate" the PNP transistor from the internal currents source when the input is taken above VCC.

    See section 2 of the following appnote. Though it is for the LM324/358 op-amp, the PNP input core is similar to the LM339/393.

    Design Guidelines for Devices with LM324/LM358 Cores