Question about opamp type?

Discussion in 'General Electronics Chat' started by samy555, Oct 27, 2013.

  1. samy555

    Thread Starter Active Member

    May 24, 2010
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  2. MrChips

    Moderator

    Oct 2, 2009
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    LM311 is not an op-amp.
    I don't know the answers for the other two questions.
    (IC1b looks like some kind of positive peak detector).
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Isn't it an Opamp, albeit a comparator?
    It is included in my Walter Jung Op Amp Cookbook;)
    Max.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    True, but an LM311 comparator will not work in this circuit.
     
  5. samy555

    Thread Starter Active Member

    May 24, 2010
    116
    3
    I thank everyone for contributing but stayed a couple of questions I hope to answer them if possible.
    thanks
     
  6. samy555

    Thread Starter Active Member

    May 24, 2010
    116
    3
    I thank everyone for contributing but stayed a couple of questions I hope to answer them if possible.
    thanks
     
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Without taking the time to analyze the circuit (I don't know much about radio circuits), I'd say D1 and D2 seem obviously meant to ensure current only flows into the base of T1, so the base-emitter junction doesn't get reverse biased.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    I'm trying to explaining those questions, but I'm not guarantee that they are all correct.

    1. This circuit need the op amp to enlarge the signal, but LM311 is not a op amp, it is a single comparator.

    2. R5 providing the current for the base of T1, why does it need D2, because the signal came from ant is a high frequency modulation signal, but the base of T1 only need the positive signal, so using D2 to cut off the negtive signal.

    But the signal still need to send to base of T1, so using D1 and R6 to cut off the negtive signal to protecting the base of T1.

    3. D3 is using as a half rectifier to cut off the negtive high frequency signal, and the c10 is a filter to clean the high frequency modulation signal, the R15 is designed to discharge for c10, so only the low frequency signal can send to the Op amp

    c7 is using for limited the oscillation.
     
  9. Slarsen

    New Member

    Apr 29, 2013
    12
    4
    I'm no RF engineer but I understand a little and have designed a little.

    Since the gain of IC1a is only 6.8M/100k = 68, there must be a heck of a lot of gain in the RF amplifier, transistor T1. Therefore T1 must be a regenerative type of receiver circuit. Capacitor C7 provides positive feedback to this amplifier section, which is what gives it a great deal of gain.

    A regenerative receiver is inherently unstable. Like when a microphone in a PA system is too close to the loudspeaker, it starts to squeal. I believe that diode D1 is used to squelch the feedback. I think that because the only way D1 can conduct (and thus have a purpose for existing) is if the voltage at the base of the transistor goes negative, and that could only happen with a fairly large AC (RF) voltage there.

    The purpose of diode D2 is to demodulate the RF signal. I know that's the case because this is a 400 MHz receiver and no op amp is gonna handle that. Therefore demodulation must occur before the op amp. So D2 is an envelope detector.

    Op amp IC1a is an amplifier with a gain of 68. It is needed for it's low output impedance so that it can drive the diode D3 circuitry. I believe the D3 circuitry is an accumulate and dump low-pass filter, something that is used to increase the gain of a receiver. Capacitor C10 accumulates a charge from current supplied through resistor R15 when the data is at one state, and then diode D3 dumps (discharges) the capacitor when the data transitions to the other state. IC1b acts as a comparator and outputs the data.

    If this circuit works reliably I'd say it's a rather ingenious design, the regenerative section in particular. I'm sure the designer is proud of himself. I would be.

    You can use an LM324 op amp for this. I'm not sure if the op amp in this circuit requires both a positive and negative power supply, or if a single supply will work. I suggest you try both. If it works one way, it likely won't the other way.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A quote from the description listed:

    LM311 is an excellent Operational amplifier capable of driving DTL, RTL, TTL, or MOS logic. The output can switch voltages to 50 volts at currents to 50 mA. IC LM311 operates between 5 and 30 volts single supply. It can be used to drive Relays, Lamps, Solenoids etc. But when compared to other Op Amps, the pin connections of LM311 are different. This design note will help you to design circuits based on LM311.

    LM311 Inputs
    Like all other single Op Amps, LM311 also has two inputs. These are Inverting input and Non inverting input. In typical Op Amps, the inverting input(negative) is pin 2 and Non-inverting(positive ) input is pin3. But is LM311, the condition is just reversed. Pin2 is non inverting input and pin3 is inverting input.

    But it won't work as a linear amplifier in this case.

    Max.
     
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