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# LM148QML: Maximum Output swing voltage

Part Number: LM148QML
Other Parts Discussed in Thread: LM148, UA741

Dear Sir,

we are using LM148 in our project and we are facing output swing issue.

It is simple inverting amplifier with resistor value 500KOhm with 100kOhm load and supply voltages +/- 11V.

Case 1 : I/P DC voltage -11V output is 10.3V

Case2 : I/P DC voltage 11V output is -8.7V... output is saturating at -8.7V

Please guide me to resolve the issue.

• Hey Suresh,

Can you please add a schematic? I know you describe your circuit but it is hard to visualize clearly.

All the best,
Carolina

• Hello caro walter,

Here is the circuit

• Hello Suresh,

I believe the circuit is working as expected, as per Figure 6 and 7 from the datasheet.

I have highlighted an output swing of 10V and -10V (instead of -11V and 11V) since it was easier. As you can see for 10V, the output source current reaches ~20mA; however, for -10V, the output sink current ~15mA.

Therefore in the scenario of the negative output swing being less than the positive output swing is expected.

All the best,
Carolina

• How should i balance the output voltage swing for Positive and Negative output voltage.

• Hi Suresh,

there's something wrong. The output voltage should be much more symmetrical.

What is connected to the output of LM148? Seems like the OPAmp sees different loads for positive and negative output voltage.

Insert a 220R isolation resistor when measuring the output voltage of LM148. A DVM (digital voltmeter) can show a considerable input capacitance, destabilizing the OPAmp. The OPAmp oscillates and the DVM measures the DC component of oscillation which can show a drastical offset.

If this does not solve the issue, take a fresh LM148 and measure again.

Kai

• when it is used as inverting unity follower, for -ve voltage input, output is saturating at 10.3V and for +ve voltage input, output is saturating at -8.7V, why this imbalanced saturation level for -ve and +ve voltage inputs.

In LM148 out of four op-amps only two op-amps are used and unused op-amps are terminated properly, requesting any technique to balance the output saturation limit for both -ve and +ve voltage inputs.

• Hello Kai,

same load is there for both positive and negative output voltage.

even with fresh LM148 also it is showing exactly same response.

• Hi Suresh,

Have you inserted the recommended isolation resistor?

Are the supply voltages stable? What do you measure there?

Kai

• -- Load is 100KOhm for both -Ve and +Ve input signal.

-- Same response even after inserting isolation resistor

-- TDK - Lambda make stable power supplies used

-- even after changing new IC too same problem( Asymmetric output voltage swing for -ve and +ve voltages)

• Hi Suresh,

why would you want to drive the output into saturation at all? Saturation voltage is not only asymmetric for both swing polarities but also alters with temperature.

What happens when you short-circuit the +input of OPAmp to signal ground?

How is the circuit behaving with +/-15V supply voltage?

Can you decrease the feedback resistances? Then you could eventually add two low leakage zener diodes.

Or have you thought about using asymmetric bipolar supply voltages?

Kai

• Hello Kai,

As per the application demand one of the parameter to check saturation limit and requirement is symmetric,

Still asymmetric only, when +input of Op-Amp connected to ground.

Asymmetric Output swing for +/-15V Supply too.

I can't decrease the feedback resistor, that will effect to other parameters and it won't make output swing symmetric

In the system(Actual Application) we have fixed symmetric supplies.

If i decrease the +11V supply to 9.5V output is symmetric, by using resistor divider but as per our application output swing should be symmetric for the power supply range from       +/- 11V  to +/-15.5V, but again output swing asymmetric for the power supply other than    +/-11V.

As per the datasheet output swing should be symmetric.

• Hi Suresh,

provided you don't want to take another OPAmp showing a more symmetric output voltage swing, you could do it this way:

suresh_lm148.TSC

The BAV99 could be replaced by ultra low leakage diode BAV199, or similar.

Kai

• Hello Suresh,

LM148 does not have a symmetric VOM.  LM148 claims to have characteristics similar to uA741 that also has an asymmetric VOM. Where in the data sheet does it claim sysmetric?

Would you like me to suggest a different QML device that has a more symmetric VOM?

• Hi Suresh,

this is another method to improve the symmetry of output voltage swing:

Kai

• Hello Ron,

In datasheet it has claimed that for +/-15V supply VOM is +/-12V, but practically we are not getting +/- 12 V with load of 100kOhm

Thanks for confirmation, Please suggest different QML device with 4Ch(4 Op-Amps) and also should be cost effective, this product will be in mass manufacturing.

Thanks

• Hi Suresh,

must the replacement also be a "QML" OPAmp?

Kai

• Yeah Ron,

Must be a 'QML" OPAMP

• Suresh,

I'll check for alternatives tomorrow. Today is USA holiday.

• okay ron

• Suresh,

The lowest price device (that works with 22V supply) with pretty good VOM matching is TLE2071MJGB (for CDIP)

The data sheet separates VOM+ and VOM- and they are close to matching. I also found test data for commercial version of the device and the matching was just 160mV off.

• Hello Kai,

above circuit suggested circuit is effecting to other parameters,

I have designed circuit, requesting you to verify and confirm the reliability of circuit performance

• Hello Kai, Can you please review above circuit.

• Hey Suresh,

Is changing the op amp to the TLE2071MJGB not an option in this case? It seems to solve the symmetrical output issue, and would be a more robust/cheaper solution than dropping the voltage over diodes.

Best,
Jerry

• Hello,

cost of TLE2071MJGB is high and also high lead time.

Requesting you to please review above circuit, by putting two no series diodes(1N914) in +ve supply part to reduce the +ve supply by 1.4V i.e, net supply voltages are +13.6 and -15V, which resolves my problem.

Let me know any technical issue/ Reliability issues with above concept.

Thanks

• Suresh,

I do not see any technical issue with the solution provided, however if you are able to increase the supplies to give more headroom to the output swing, then I don't see why you need the diodes there. Does the design work with ±15V supplies and no diodes?

Best,
Jerry

• Jerry,

design requirement is symmetric maximum output swing with power supply ranges from ±11V to ±15V, Op-amp configured as Inverting unity gain amplifier.

Our concern is reliability issue, as I am putting diodes in power supply path and this design suppose to undergo Screening Test for 250Hrs at 70degreeC with power supply ±15V,

• Suresh,

This should work so long as the max ratings of the diode are not exceeded. There will be some variation between diodes in voltage drop and some variation in the max output swing between devices but if this variation is within acceptable limits for being symmetrical you should be okay. This is within normal operating conditions of the device

Best,
Jerry