H Bridge Amplifier

In this blog, I will be selecting an MOSFET n and p channel H Bridge. I selected this because of the simplicity of the end design.

First I go to Digikey, go to the product index and select in stock items. Next I go to Discrete Semiconductor Products and select FETs arrays. Under FET type, I select 2 N and 2 P-Channel H-Bridge. I need a device rated for 2 Amps. The device rated for 3.98A is $1.14. One rated for 2.7A is $1.93. A few rated for a lower amperage are even higher. The choice is clear, if I pick the ZXMHC3F381N8TC, the MOSFET Complementary H-Bridge, 30V, 8-SOIC. The best part is the switching speeds. The turn on delay time is 2.5 ns. The turn off delay time is 11.5 ns. This is much faster than both of the preamplifiers.

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Electrical Design Engineer, Minneapolis, Minnesota

pMOSFET preamplifier

I am designing a driver for an H Bridge amplifier. I want the amplifier to be driven by two PWM signals, and be capable of being completely off if no signal is present. This is possible if each of the four H-Bridge inputs are taken as independent, with the two inputs at the opposite corners as binary opposites of each other.

Using the MOSFET preamplifier, I have one quarter the signal I need. The other quarter I get from logically inverting the signal. I can do this in one of many ways, the most common is using another n or p channel amplifier. Using an n channel amplifier, like I did in the previous post, might be the most common. The n and p channel MOSFET has many different characteristics, which makes it more difficult to incorporate in the design. If I wanted the total current consumption to be minimal during off time, I would use a pMOS to impelement the second amplifier, like the following picture.

I could use two n channel MOS transistors, to achieve the same voltages. The reason I like this better for some applications is because of the region of operation. PWM low is no signal. Both T1 and T2 are in cut off when the PWM is low, in this configuration. Zero is the most common signal for this application. It would make sense to have as little current as possible when the device is considered off. Being a conscientious engineer, I think it important to consider these options.

I have chosen the SI2301. It has a footprint similar to that of the one I chose for the n channel amplifier. They both are for high speed switching, very low threshold, and useful as a general purpose switch. Rds_on are different. The switching speed is a touch slower, allowing for a 15 MHz PWM speed. Having only an n channel amplifier set up would allow for a 37 MHz speed.

So this is my trade off, keep speed high and maintain some simplicity, or be able to keep the current level really low when the device is not receiving any signal. The thing is, I am purposefully keeping the current very low, to minimize any drain source voltage, so is this an actual advantage?

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Electrical Design Engineer, Minneapolis, Minnesota

MOSFET Preamp

In this article, I will talk about some of the needs I have for this amplifier, some of the decisions I made in choosing devices, and how it fits in this application.

I want to amplify the voltage of a signal, which is commonly called a preamplifier. The signal is from a PWM output from an MCU. This means only two values are important, logic high and logic low. This also means that amplifier linearity is not important. I also wanted the input impedance and voltage gain to be relatively high. Most importantly, I wanted the system to be very simple.

I knew an N-Channel Metal Oxide MOSFET would be most ideal for this application. They can have a very fast switching time and have a high input impedance. The question was, how do I choose which device to implement. Digikey made it fairly straightforward. They had a column for the Threshold Voltage. I knew the target voltage I wanted was 3.3 V. I needed Vth to be well below that. I found one with a threshold voltage of 0.4 Volts. BSH103,215

I looked at their datasheet, and saw a range of currents that was well beyond the scope of what I was trying to do. For a preamp, I don't necessarily need the current to be very large, in fact it is better if it isn't. I can limit it to a small value by including a large resistance.

This is what I wanted. When the input is 0, I want the output to have 16 Volts. When the input is 3.3, I want the output to be very close to zero. This is impossible with nearly all amplifiers, but with device comes very close to that possibility.

Notice that the Current is in Amps. Also notice, the further up you go in current, the further away the Voltage strays from zero. So, if I want the output Voltage to be approximately zero, I need to keep the current to a minimum, close to 1 mA.

Therefore, I choose this device, with a 16 kilo Ohm resistor connected to the drain, and 16 Volts connected to that as my amplifier.

One thing of concern is the maximum continuous current, the switching time, and the maximum power allowed. The current is well below that, so that isn't an issue, but it does limit other potential applications. The difference between when the input changes and the output changes is a major concern for PWM applications. The switching time is 20 to 27 ns, which means the fastest the PWM could switch is 50 to 37 MHz. The maximum power rating is 540 mW. The drain leakage current is 100 nA, the drain voltage at that time would be 16 V. This results in 16 micro Watts of power. At the other extreme, the current is 1 mA, the drain voltage is approximately 0.1 V. This results in 0.1 mW of power. These are both well within the max rated power rating, but more questions remain. What happens when the device switching? This can only be answered accurately by wiring up the device and testing it using an oscilloscope for current and voltage while in application.

This completes my design of a MOSFET preamplifier.

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Electrical Design Engineer, Minneapolis, Minnesota

Piccolo Boot ROM

This is what I need to work with the boot ROM in the TI Piccolo C2000 devices.

The Boot ROM is factory-programmed with boot-loading software. Boot-mode signals are provided to tell the bootloader software what boot mode to use on power up. The user can select to boot normally or to download new software from an external connection or to select boot software that is programmed in the
internal Flash/ROM. The Boot ROM also contains standard tables, such as SIN/COS waveforms, for use in math-related algorithms.

Section 3.5.11 is a security section.

When the emulator is connected, the GPIO37/TDO pin cannot be used for boot mode selection. In this case, the boot ROM detects that an emulator is connected and uses the contents of two reserved SARAM locations in the PIE vector table to determine the boot mode. If the content of either location is invalid, then the Wait boot option is used. All boot mode options can be accessed in emulation boot.

The default behavior of the GetMode option is to boot to flash. This behavior can be changed to another boot option by programming two locations in the OTP. If the content of either OTP location is invalid, then boot to flash is used. One of the following loaders can be specified: SCI, SPI, I2C, CAN, or OTP.

Page 34, 35 tms320f28068.pdf

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tms320f28068 JTAG

One thing I find missing from embedded digital design is simplicity. One thing I would like to do is boil a complex design down to some key components. First up is JTAG. This is one way an engineer or technician can interface with a embedded microcontroller, that is already connected to a circuit.

I am focusing on Texas Instruments c28x Piccolo MCUs. All information is from the TI website and relevant datasheets.

Pull up resistor means there is a resistance connected between it and the supply voltage, and pull down resistor means it is connected to ground. They are internal when using JTAG.

/TRST has a 2.2 k Ohm pull down resistance.

On the 2806x device, the JTAG port is reduced to 5 pins (TRST, TCK, TDI, TMS, TDO). TCK, TDI, TMS and TDO pins are also GPIO pins. The TRST signal selects either JTAG or GPIO operating mode for the pins. During emulation/debug, the GPIO function of these pins are not available. If the GPIO38/TCK/XCLKIN pin is used to provide an external clock, an alternate clock source should be used to clock the device during emulation/debug since this pin will be needed for the TCK function.

The headers are JTAG-based emulators - XDS510™ class, XDS560™ emulator, XDS100.

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Page 22, 33, 54, 69, 136 on the tms320f28068 datasheet.

audio amplifier

Audio signals are by default oscillations from a common center. One supply designs are therefore limited or impractical. I think the best way to keep the design simple is to add a DC bias to the signal.

I am considering using either a common emitter or a voltage follower BJT amplifier, with a capacitance coupler and two biasing resistors.

The human threshold of hearing is 16 Hz to 20 kHz. This creates a complication for biasing a BJT using two resistors and a coupling capacitor. The equivalent resistance of a capacitor at a given frequency is 1/(2 pi C f) The equivalent resistance of a 0.1 uF capacitor at these frequencies is 100 k and 80 Ohm, respectively. The highest ceramic capacitor is 220 uF. The equivalent resistance for that is 45 and 0.04 Ohm. The higher the capacitance, the less spread the equivalent resistances will be. The problem with going this route is that now the DC voltage will have a low resistance to the signal source, which is something I want to avoid.

To have the widest range of usability, I should use a stand alone analog to digital converter. In Digikey, under the heading Integrated Circuits (ICs), Data Acquisition, Analog to Digital Converters, 8 bits, 100k to 1M Sampling Rate (Per Second), and Dual +- Voltage Supply Source. This ends up being MAX152 to MAX156. More on them later.

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speaker amplifier 1

As a start, I think it would be best to start with a midrange speaker set. I am considering Sony SS-F5000 Floor Standing Speakers. They come in pairs.

• Frequency Response : 45 - 50,000 Hz
• Impedance : 8 Ohms
• Mid Range Size(s) : 3 ¼” Midrange Driver
• Power Requirements : 150W
• Quantity (Packaged) : 2
• Quantity of Tweeters : 1
• Quantity of Woofer(s) : 2
• Sensitivity : 88 dB
• Speaker Terminal Type : Binding Post
• Tweeter Size : 1" (2.5 cm)
• Woofer Size(s) : 8" Woofer

A 150 Watts and 8 Ohm impedance results in a peak amplitude of 4.33 Amperes and 34.6 Volts. The frequency response is higher than I would like, but it will do for the price.

My whole goal is to do away with a lot of the necessary components needed for audio amplification. A receiver can be a very complex and pricey piece of equipment. You get what you pay for. Sometimes it is nice to have a lot of bells and whistles, but sometimes you just want high fidelity. I am going for fidelity.

It is simply connected to a computer with a head phone jack. The audio signal is digitized. The 8-bits are sent to the speaker site using a parallel port. Because the signal is digital, the distance possible before high distortion is greatly increased. The speakers are given a pulse width modulated carrier signal, which is filtered.

Keep tuned.

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Electrical Design Engineer, Minneapolis, Minnesota