Berkeley Electronic Interfaces Course – Robot Module 3.3 – Amplifier (Speaker Driver & Choosing Amp)

In the previous chapter, we made the robot can hear us. And now we try to let the robot be able to make some noise. Thus we will try to connect a speaker to it.

Suppose we have a small 8 ohm speaker needed to put on the robot. Apparently there are several ways to simply do that, we will start trying some simple ones and see if it works.

Method 1 – Direct drive by the MSP430

As discussed in the course that while doing a setup like this, due to the low impedance of the speaker, with 3.3V voltage supply from the board, it will theoretically drain around 412.5mA. That’s a huge a mount of current. However if we look at the MSP430 datasheet it says there’s only a 48mA maximum output on all the output pins. So when driving the speaker directly with MSP430, not only it will not be very loud, also it will make MSP won’t able to do anything else on any of the output pins due to the speaker is sucking up all the current.

Pros:

• Simple setup

Cons:

• Drain too much power or current from MSP430 board
• Might not be able to drive the speak loud enough (48mA limit due to MSP430 spec)

One way to solve this is to use an amp and let the amp provide the power to the speaker. And here is a way to do it.

Method 2 – Non-Inverting Amplifier Drive (OPA2344)

By looking at the circuit we could calculate by using the 3 golden rules, we could know theoretically the output of the amp should be with a gain of 2, which is around 6.6V, and hopefully driving the speaker with 825mA. That should sound very loud. However when you test it, as they did in the course video, you will see that the speaker sounds no much difference. The reason is that for some low power amplifiers, they have a relatively large output impedance, or in other words, they have a relatively low short current – a current at output when you short the output to the ground. And this short current is the maximum current this amp can deliver to the load on output. On the OPA2344 datasheet it notes the short-circuit current is around 15mA. No more than our previous direct drive method.

Pros:

• Low power consumption (high impedance between inputs and low quiescent current 150uA)
• No power drain from MSP430

Cons:

• Low short-current (15mA), output power low
• Consume a little power at the grounded R1

So an easy way to solve some of the low output issue above is by using a type of amps especially designed to dive speakers, an amp with low output impedance and high short current.

Method 3 – Audio Amplifier Drive (LM380 Drive)

For example if we can use LM380-8 amp as a comparator to drive the speaker, shown above. This for sure can drive the speaker very loud with a short current of 1.3A. However there are some draw backs of this amp in our application due to it’s power consumption of the battery could be really high. This is due to the relatively low input impedance comparing with the previous OPA2344 amp. The quiescent current is 7mA for this amp and only 150uA for OPA2344. (As long as I remember the quiescent current is the current that flows between the non-inverting and inverting inputs, or it could be also described as the current an amp is “naturally” drawing without and load connected. The higher the input impedance, the lower of the quiescent current, thus lower power consumption.)

Pros:

• High short current 1.3A, powerful output
• No power drain from MSP430

Cons:

• High power comsumption – Quiescent current typical 7mA

So what we do? Well practically we don’t need to make the speaker sound extremely loud in our robot case, so method 2 will more or less do the job. However by noticing the power drain on the R1, we then could just change the circuit without using feedback resisters, but using a direct feedback to so make a voltage follower.

Method 4 – Voltage Follower Drive

In this way the output voltage has no gain and we still can output 15mA mostly to the speaker and this sounds quite alright. Also in this way the power consumption is extremely low among other methods shown here.

Pros:

• Ultra low power consumption
• No power drain from MSP430

Cons:

• Low output power, but for our case it is acceptable

I believer there are more methods out there and could be discussed to improve the design. But since the learning level of this course is not aiming at those advanced topics and the method 4 will just do fine for our robot, we just decide to go on with it now 🙂