LM4562, LME49710, LME49720 Start-up Behavior
Posted: Fri May 09, 2014 12:31 pm
I became curious about a few reports on the web regarding single-supply start-up of the LM4562 and the identical LME49720 and single LME49710.
I initially suspected a common mode limitation or polarity reversal but quick tests confirmed this was not the case.
The most recent post was a guitar input using a LME49710 which quite likely had a high-value bias resistor to prevent pickup loading, some form of rail-splitter, AC-coupling and 9V operation. The schematic was not provided. Since it has a similar input I decided to plug an LME49720 into The Waveulator's front-end replacing a TL072.
I realize that the noise current of the LME49720 operating into a 1M Rbias resistor might be excessive and not the best choice for a guitar front-end but it made a good test circuit that was similar to the poster's.
This stage is not a follower: It has 6 dB of AC non-inverting gain. The DC gain however is unity.
The schematic is similar to the one here: viewtopic.php?f=6&t=606
Sure enough, just like the post elsewhere, the LME49720 latched-up on start-up.
Lowering the bias resistor value to 100K, from 1M, was the only way I found that would prevent it.
Conditions During Latch-Up
V+ is 9V, V- ground.
During latch-up:
In+ is +926 mV
In- +950 mV
Out is +827 mV.
If the circuit is latched-up, temporarily lowering the bias resistor from 1M to 100k will unlatch it.
The LM4562 and LME49710-family Have Unusually Low Input Bias Currents for a Bipolar Input Audio Op Amp.
The typical Ib is 10 nA, the maximum Ib for the LME49710 is 72nA.
These are outstanding specifications.
To put it in perspective, the NE5532 has a 1 uA typical Ib.
The datasheet for the LME49710 has no sign for the bias current; for PNP inputs the bias current should be flowing out of the IC.
The lack of a sign for the bias current specification suggests that the LM4562-family uses some type of bias current compensation.
US Patent 7649417 issued to National Semiconductor may provide clues:
http://www.waynekirkwood.com/images/pdf ... Op_Amp.pdf
With Low Bias Current Why are High-value Bias Resistors Causing Latch-Up?
Low bias current should permit higher values of bias resistors.
10 nA across 1M produces only 10 mV of error across the bias resistor.
This is an insignificant I*R drop in the bias resistor of an AC-coupled preamp.
It appears that during start-up however the bias current is significantly higher.
100K works: 1M doesn't.
What may be happening is that the initial bias current for the input pair has to be satisfied externally until the compensation circuit kicks in.
Once the initial start-up bias current requirement is satisfied, the quiescent bias current is quite small.
What Are the Limits to Values of Rbias?
I built the following test circuit and admit testing only ONE LME49710 sample.
The results however correlate closely to a different sample - an LME49720 - in the guitar input.
The results may not be indicative of a broader population of parts but the results are instructive.
Test Circuit LME49710 Bias Current Start-up
In the circuit above the V+ and V- lines are switched simultaneously.
Though shown as a dual supply circuit it is identical to a single-supply circuit with ground serving as the rail-splitter.
The bypass capacitor is 100 nF.
A hard-wired follower is shown: A follower constructed with a 1K RfB||20 pF had slightly worse start-up performance. (20 pF was required for stability.)
TP2 is monitored during start-up, TP1 is for future use to measure the magnitude of Ibias during turn-on.
The Results
The first thing I found was a supply voltage sensitivity.
Lower voltages required lower values of Rbias (Rb) for proper start-up.
Rb vs +/-V vs Latch-Up:
100K +/-2.5V Latch
100K +/-3V Latch
100K +/-3.5V OK (note 7V is near a 9V battery's end-of-life)
100K +/-4.5V OK (9V single-supply)
1M +/- 4.5V Latch (9V)
1M +/-6V Latch (12V)
1M +/-15V Latch (30V)
At the minimum supply voltage of +/- 2.5V:
10K +/-2.5V OK
47K +/-2.5V OK
68K +/-2.5V Latch
Conclusions:
The LM4562, LME49710, LME49720 and LME49740 offer excellent performance as an NE5523 and NE5534 replacement.
The designer applying them should be aware however that the low bias currents offered do not necessarily permit impedance scaling to the point that start-up bias currents, which appear to be several times higher than the quiescent, are not satisfied.
I initially suspected a common mode limitation or polarity reversal but quick tests confirmed this was not the case.
The most recent post was a guitar input using a LME49710 which quite likely had a high-value bias resistor to prevent pickup loading, some form of rail-splitter, AC-coupling and 9V operation. The schematic was not provided. Since it has a similar input I decided to plug an LME49720 into The Waveulator's front-end replacing a TL072.
I realize that the noise current of the LME49720 operating into a 1M Rbias resistor might be excessive and not the best choice for a guitar front-end but it made a good test circuit that was similar to the poster's.
This stage is not a follower: It has 6 dB of AC non-inverting gain. The DC gain however is unity.
The schematic is similar to the one here: viewtopic.php?f=6&t=606
Sure enough, just like the post elsewhere, the LME49720 latched-up on start-up.
Lowering the bias resistor value to 100K, from 1M, was the only way I found that would prevent it.
Conditions During Latch-Up
V+ is 9V, V- ground.
During latch-up:
In+ is +926 mV
In- +950 mV
Out is +827 mV.
If the circuit is latched-up, temporarily lowering the bias resistor from 1M to 100k will unlatch it.
The LM4562 and LME49710-family Have Unusually Low Input Bias Currents for a Bipolar Input Audio Op Amp.
The typical Ib is 10 nA, the maximum Ib for the LME49710 is 72nA.
These are outstanding specifications.
To put it in perspective, the NE5532 has a 1 uA typical Ib.
The datasheet for the LME49710 has no sign for the bias current; for PNP inputs the bias current should be flowing out of the IC.
The lack of a sign for the bias current specification suggests that the LM4562-family uses some type of bias current compensation.
US Patent 7649417 issued to National Semiconductor may provide clues:
http://www.waynekirkwood.com/images/pdf ... Op_Amp.pdf
With Low Bias Current Why are High-value Bias Resistors Causing Latch-Up?
Low bias current should permit higher values of bias resistors.
10 nA across 1M produces only 10 mV of error across the bias resistor.
This is an insignificant I*R drop in the bias resistor of an AC-coupled preamp.
It appears that during start-up however the bias current is significantly higher.
100K works: 1M doesn't.
What may be happening is that the initial bias current for the input pair has to be satisfied externally until the compensation circuit kicks in.
Once the initial start-up bias current requirement is satisfied, the quiescent bias current is quite small.
What Are the Limits to Values of Rbias?
I built the following test circuit and admit testing only ONE LME49710 sample.
The results however correlate closely to a different sample - an LME49720 - in the guitar input.
The results may not be indicative of a broader population of parts but the results are instructive.
Test Circuit LME49710 Bias Current Start-up
In the circuit above the V+ and V- lines are switched simultaneously.
Though shown as a dual supply circuit it is identical to a single-supply circuit with ground serving as the rail-splitter.
The bypass capacitor is 100 nF.
A hard-wired follower is shown: A follower constructed with a 1K RfB||20 pF had slightly worse start-up performance. (20 pF was required for stability.)
TP2 is monitored during start-up, TP1 is for future use to measure the magnitude of Ibias during turn-on.
The Results
The first thing I found was a supply voltage sensitivity.
Lower voltages required lower values of Rbias (Rb) for proper start-up.
Rb vs +/-V vs Latch-Up:
100K +/-2.5V Latch
100K +/-3V Latch
100K +/-3.5V OK (note 7V is near a 9V battery's end-of-life)
100K +/-4.5V OK (9V single-supply)
1M +/- 4.5V Latch (9V)
1M +/-6V Latch (12V)
1M +/-15V Latch (30V)
At the minimum supply voltage of +/- 2.5V:
10K +/-2.5V OK
47K +/-2.5V OK
68K +/-2.5V Latch
Conclusions:
The LM4562, LME49710, LME49720 and LME49740 offer excellent performance as an NE5523 and NE5534 replacement.
The designer applying them should be aware however that the low bias currents offered do not necessarily permit impedance scaling to the point that start-up bias currents, which appear to be several times higher than the quiescent, are not satisfied.