OP27: Amplifier NPN output stage, amplification factor

Part Number: OP27

Hello,

comparing the internal schematic of the TIs OP27 shown in the data sheet with the OP27 of Analog Design one can see that the TIs OP27 Amplifier is depicted with a pure NPN bipolar transistor output stage where the ADs shows a complementary design in its data sheet. This difference would yield a voltage amplification factor higher than 1 (how much ?) and a signal inversion at the output pin with the TIs design..

A higher voltage amplification factor of the output driver stage would yield smaller voltages and voltage stress over the internal frequency compensation MOS-capacitors, which can be favorable.

True ? 

Sincerely, Christian Daniel

3 Replies

  • Hi Christian

    I had to find someone to ask one of our designers who is familiar with the legacy bi-polar process. My question was that ADI shows a complimentary output and we have an all NPN output and was that legitimate and here is the answer I got.

    > This is a legit difference between the parts caused by our process 435 pnp’s being too slow be used in the signal path (due to low ft). For that reason most of our op amps on this process use a modified ALL-NPN output stage, including Q45, shown in our OP27 datasheet. However, this fact does NOT cause any meaningful difference in the open-loop gain because the last stage gain is a function of the parallel combination of gains of npn and pnp transistor (G=~(re||Ro)/re) and the lower one (re of Q46 npn transistor) dominates the overall gain.

    This means that the NPN transistor will dominate the gain path for both the ADI solution and the TI solution so they should perform very closely to each other in spite of this difference.

    Hope this helps you!

    Regards,
    Christian
  • In reply to Christian Yots:

    Hello Christian,

    Many thanks, all NPN output stage convinces me ! This means there must be an inversion inside the output stage wherefor the connections of the bases of Q21 and Q22 must be exchanged. In the schematic in the datasheet both transistors Q21 and Q22 are connected to the same collector of the input transistors - correct ?

    Q3 shows an inverted NPN - intention ?

    Does the minor internal changes of the OP27 have any (positive)  effects on the Singe Event Gate Rupture (SEGR)  hardness of the internal feedback MOS-capacitors ?

    Greetings, Christian Daniel

  • In reply to Christian Daniel:

    Hi Daniel,

    After double checking, we confirm the output is, in fact, complementary. We do not have an explanation for why the published schematic was used but we do agree it is incorrect. We will update the datasheet to reflect the corrections but this will take some time.

    Thanks, Christian