A bridge-tied load (BTL) amplifier circuit doubles the output voltage (and potentially quadruples the power, output current limits notwithstanding) by using one "master" opamp for gain followed by (i.e cascading its output into) another "slave" opamp with unity gain but used in inverting config. This is type of cascading BTL circuit is illustrated for example in Fig. 1 in Maxim app note 1122 and in Fig. 73 (on p. 24) in the OPA454 datasheet (OPA454 being TI's highest voltage opamp it is somewhat natural that its datasheet is replete with circuits for increasing output voltage swing even further.) Not all opamps are unity-gain stable (unless externally compensated), but it is possible to adapt the cascading BTL circuit for opamps that aren't unity-gain stable (without adding external compensation) by feeding only a portion of the master opamp's output voltage to the slave opamp through a simple voltage divider. For example, Marston's Audio IC Circuits Manual shows such a circuit using TDA2030 (which is not-unity gain stable) in Fig 4.28 on p. 108. So lack of unity-gain stability of the opamp is not a show stopper for the cascaded BTL approach, although it does increase component count slightly.

On the other hand, a non-cascading BTL circuit can also be built simply by feeding the input signal into one inverting and one non-inverting opamp in "parallel" [sub]circuits and adequately changing the gain (sharing it) between opamps. According to the aforementioned Maxim app note 1122, which shows such a circuit in its Fig 2., that's actually a better approach than the cascaded BTL because "Both [opamps] amplify the same input signal, so neither one reproduces noise, distortion, or clipping introduced by the other". According to the same Maxim app note, a non-cascaded BTL circuit has additional advantages in the case where the availability of a single-rail power supply (such as car audio etc.) dictates that the opamp inputs additionally need to be DC biased (for AC-coupled [that is capacitor-coupled] inputs); chiefly among the advantages claimed in this single-rail supply case, it is said that the non-cascaded BTL circuit "eliminates signal injection into the high-impedance bias source".

After having heard all praises lavished on the the non-cascaded BTL topology in Maxim's app note (that other sources seem have missed), my question is: what are the disadvantages of the non-cascading BTL circuit relative to the more typical (judging by the sources I've surveyed) cascaded BTL? (And, sub-textually, why is the typical BTL opamp-based circuit exemplified using the cascaded approach?)


1 Answer 1


Nice, (+1 for opa454, I didn't know this opamp.) I've also used BTL to get more voltage out of low voltage high current opamps. (OPA569 driving a TEC.) As far as your question. I guess the traditional approach doesn't have any gain balancing. (It's just a unity gain inverter.) And I've never been worried about noise in the driver stage. So my only answer is simplicity.

  • \$\begingroup\$ By "the traditional approach doesn't have any gain balancing" I'm assuming you mean the cascading approach doesn't have worry about that. I think the parallel/non-cascading BTL would indeed have to worry about well matching the gain on the two opamps (via the resistors) or else even with sine wave as input you can end up with an (average) DC bias on the load. (Continued in my next comment.) \$\endgroup\$ Commented Dec 9, 2014 at 22:23
  • \$\begingroup\$ The lack of worry about gain matching in the cascaded BTL holds however only if the (slave) opamp is unity gain stable. If instead you have to take only a fraction of the master's output voltage and feed it to the slave (as in the TDA2030 bridge circuit from Marston's), then you do have to worry about the precision of that divider, because it will affect the swing of the slave (in conjunction with its own feedback) so it shares the same potential problem of unequal swings in the two opamps as for non-cascaded topology. \$\endgroup\$ Commented Dec 9, 2014 at 22:29
  • \$\begingroup\$ Yeah the opamp has to be unity gain stable, I think I made mine both unity gain... maybe a gain of two on the first stage with a little integrating cap? ..you'd like the 2nd stage to be a bit snappier (faster) than the first, so it can follow. Maybe that's a down side the transient / step response, (I've never really looked.) (most of the step response of the TEC was the opamp hitting and coming off the rail... that's the Big step response. :^) \$\endgroup\$ Commented Dec 9, 2014 at 23:32
  • \$\begingroup\$ Have you looked at the LTC6090? \$\endgroup\$ Commented Dec 10, 2014 at 0:07
  • \$\begingroup\$ Yeah, I've looked at its datasheet not so long ago. It seems to be Linear's answer to OPA454; if you actually want to use LTC6090, be sure grok its SOA because at its highest supply voltage it can only output a fraction of its "typical" current. But as far as my question is concerned (which is applicable to any opamp not just the esoteric high-voltage ones) LTC6090's datasheet doesn't seem to hold any clues; there isn't a single BTL circuit in that datasheet, as far as I can tell. \$\endgroup\$ Commented Dec 10, 2014 at 0:35

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