# BTL Amplifier... Where Does the Current Go?

#### dannybeckett

Joined Dec 9, 2009
184
Hi Guys,

I'm designing a bridge-tied load amplifier where both speaker terminals are driven by amplifiers that are 180 degrees out of phase with eachother. Powering these amps are two SMPS connected in series, with the middle connection grounded, creating -25V, 0V and +25V.

Now if we think about a positive 5V DC signal flowing through this system into a resistive load - we have current coming out of the +25V rail, through the non-inverting amp and into the load. Then it comes back into the inverting amp (which is outputting -5V), and sinks into the -25V rail. The strange thing here is that none of the current has returned through the 0V grounded connection between the two series SMPS. Is this right? If so, the current coming out of SMPS A positive rail actually retuns through SMPS B negative rail... Is this OK to happen? It feels like some rule is being violated here.

Reason I'm asking is because when it comes to laying this amplifier out, I'd like to know whether I need to create thick 0V tracks back to the SMPS's, or whether it's only the +ve and -ve rails that will be handling all the current.

Cheers!

Dan

#### AlbertHall

Joined Jun 4, 2014
12,073
That current flows through the connection between the two SMPS that puts them in series.

#### dannybeckett

Joined Dec 9, 2009
184
That current flows through the connection between the two SMPS that puts them in series.
So to finish the analogy, the current goes into the -25V rail (SMPS B -ve), out of SMPS B +ve (ground) terminal, into SMPS A's -ve (ground) terminal, thus closing the loop?

This makes a lot of sense now! Thank you for clearing that up

Last edited:

#### AlbertHall

Joined Jun 4, 2014
12,073
So to finish the analogy, the current goes into the -25V rail (SMPS B -ve), out of SMPS B +ve (ground) terminal, into SMPS A's -ve (ground) terminal, thus closing the loop?

This makes a lot of sense now! Thank you for clearing that up
Zackly!