RF Input and Amplification

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					RF Input and Amplification

Basic Input Circuitry
Rockford’s engineering technologies, applied to today’s amplifiers, have made them quite rugged while being flexible, reliable, and efficient. The input section of these amplifiers is now built with a variety of options that allow for easier installation and versatility. With surface mount topology, more of these options can be implemented into a smaller space resulting in sound that is crystal clear with virtually no cross-talk. Among these features to be discussed are:  Trans*ana circuitry  Input connectors and their various uses  The use of operational amplifiers  Crossovers

Patented Circuitry
Trans*ana (Trans-conductance Active Nodal Amplifier) circuitry is an efficient, short loop amplifier design using Vertical MOSFET output transistors. The input and predriver stages operate at low voltage while the output MOSFETS are connected in a source-on-rail (high voltage) configuration to deliver their full voltage gain.

Input connectors
RCA input jacks: As the industry standard, RCA jacks provide an easy connection for signal level input. They are gold plated to resist the signal degradation caused by corrosion. RCA pass-thru-jacks: These jacks provide a convenient source for daisy-chaining an additional amplifier without running an extra set of RCA cables from the source unit or pre-amplifier. High Level inputs: These inputs use a detachable connector terminated with 20 AWG leads. These inputs should be used if the source unit has only speaker line (high level) outputs and not RCA outputs. Loss of signal or degradation of the signal input can be attributed to broken input jacks and attention to their condition is a valuable key in troubleshooting intermittent signal problems. The following figure shows the schematic of the typical use of all three types of inputs as applied to Rockford amplifiers. Note the use of a buffer op-amp to either amplify or attenuate the signal depending on which input is employed. Signal input switches allow amplifiers with 4 or more channels to be driven with either 2 or 4 inputs. The Punch 600a5 5-channel amp allows the amp to be driven with 2,4, or 6 input lines.


figure 1. Configuration of inputs with the use of a buffer

Operational amplifiers
The operational amplifier, or op-amp for short, is a pre-built amplifier module in an IC package designed to be inserted into almost anywhere an amplifier is needed in a circuit. An op-amp has an inverting input (marked with a – sign) and a non-inverting input (+) with a single output. Most analog applications use an op-amp that has some amount of negative feedback. This feedback tells the op-amp how much to amplify a signal. An op-amp with no feedback will not amplify and is said to have unity gain. A unity gain arrangement is also called a Voltage follower since it tracks the input voltage at the exact same level at the output. Unity gain op-amps are an excellent choice for use in comparator circuits. Depending on the circuit, the input signal may or may not need to be inverted. Inversion of the signal is accomplished by placing the signal into the inverted input, thus causing the output to be 180 degrees out of phase making it useful in bridging circuits.

figure 2. Schematic symbol and layout of an op-amp 3

If an op-amp is to be used as an amplifier, a resistor is placed on the input and a corresponding resistor is placed in the negative feedback loop, usually in a set ratio between the two. For example, to make the op-amp double the input signal on the output, the feedback resistor will have to be twice the value of the input resistor. The following figure represents this operation.

figure 3. 2X non-inverting amplifier

If R1 has a value of 10kohms, then the value of R2 will have to be 20kohms. The opamp is said the have a 2:1 ratio, or known as a 2x amplifier. If however, the input resistor is higher than the feedback resistor, then the op-amp will attenuate instead of amplifying. This allows the op-amp to be used as a buffer in the circuit. A buffer is an isolating circuit interposed between two circuits to minimize reaction from the output to the input. It usually has a high input impedance and low output impedance. It may be used to handle a large fan-out or to convert input and output voltage levels. Refer to figure 1. If an adjustable gain is needed, it is only a matter of providing a way to alter the ratio of R2 to R1. The use of a potentiometer (variable resistor) is employed. The pot is always wired up with the wiper arm connected to one side of the pot itself in case that the wiper arm ever fails to make contact (this will prevent the feedback loop from ever being able to open up). In the following figure, with R1 a 10kohm and R2 a 100kohm potentiometer, the op-amp will have a gain of 0 to 10x.

figure 4. Adjustable gain op-amp


Circuits that implement this type of op-amp/potentiometer combination in a Rockford power amplifier are the Input gain and Punch Bass boost circuits. The gain circuit may employ a single pot/op-amp for a single channel or mono amp, or a multi-potentiometer with several op-amps for 2 or more channels. The Punch Bass boost circuit helps correct acoustical deficiencies in the listening environment by helping reproduce full range sound without adding excessive boost – the punch bass control is a narrow band adjustment band centered at 45Hz variable from 0dB to +18dB. Some of the amplifiers use a board-mounted pot while others may use a remote potentiometer. In short, it is used as a basic tone control circuit.

Crossovers and their uses
The human audio range is from 20Hz to 20kHz – a ratio of 1:1000. None of the existing speaker drivers made can cover the full range of sound frequencies with acceptable quality. A crossover’s job is to divide the full frequency range into a few narrow bands and a dedicated driver will reproduce each frequency band. There are two common types of crossovers, “passive” and “active”. Both types of crossovers consist of a single (or group) of electronic parts responsible for dividing or blocking frequencies. Once the frequencies are divided, they are routed to the correct speaker. Passive crossovers consist of passive components (inductors, capacitors, resistors) and are installed after the amplifier, just before the drivers. Active crossovers consist of active components (IC’s, transistors) and are installed before the power amplifier or on the amp’s input stage. Crossovers are considered to be filters that will transmit frequencies within certain designated ranges (pass bands) and suppress signals of other frequencies (attenuation bands). The frequencies that separate the pass and attenuation bands are called cut-off frequencies. This is usually the 3dB down point from the signal level. Filters are classified according to the ranges of their pass or attenuation bands as low pass, high pass, band pass, and band stop. The term “slope” regarding crossover networks is the amount of attenuation (reduced output) or “roll-off” the crossover has above or below the crossover frequency. The slope of the crossover determines how quickly the information is attenuated. The steeper the slope, the higher the number, the faster information is attenuated. A 12dB per octave crossover slope will not cut off as much information as fast as a 24dB/octave crossover slope. The figure below shows how the level of audio signal rolls off after passing through different types of crossovers.


Rockford amplifiers use an internal 12dB/octave Butterworth filter selectable for High pass (HP), Full range (Full), or Low pass (LP) operation. Some amps use a fixed 80Hz LP/120Hz HP crossover (switch-able) while others use a crossover potentiometer variable from 50Hz to 210Hz. In addition, the Punch 600a5 sub channel operates on a 24dB/ octave crossover.

Signal Routing
The following figure shows the route an audio signal takes through the input section of an amplifier to the amplification stage and ultimately to the speaker to produce sound.

figure 6. Audio signal path

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Amplification Stage
Rockford amplifiers are of a class AB type. A class A amplifier is a linear amplifier where the output current flows over the whole of the input cycle. These amplifiers have low distortion but low efficiency. Distortion can occur with large signal operation due to the device transfer characteristics becoming nonlinear. A class B amplifier is a linear amplifier operated so that the output current is cut off at zero input signal and a half-wave rectified output is produced. Two transistors are required in order to duplicate the input waveform successfully, each one conducting for half of the input cycle (push-pull operation). Class B amplifiers are highly efficient but suffer from crossover distortion and must be properly biased. The output devices, especially MOSFETS, require a turn-on current of approximately 50mA per device to prevent having to be turned on and off constantly, eliminating the zero-cross distortion. Class AB amplifiers give the best of both class A and class B amplifiers. A class AB amplifier is a linear amplifier in which the output current flows for more than half but less than the whole of the input cycle. At low input signal levels class-AB amplifiers tend to operate as class-A and at high input signal levels as class-B.

As previously discussed, the Switching Delay voltage, from the turn-on circuitry, is controlled by the LM339 Quad comparator. This signal switches on the current source transistor to the input differential pair amplifier. The current source transistor will pull an equal draw of current through each transistor of the differential amplifier and through each of the transistors of the current mirror. Any difference in the amount of current between the differential pair will result in an inability to pass a signal. The purpose of the current mirror is to reverse or mirror the signal of it’s input. See figure 7

figure 7. Input to the amplifier section

For a left channel stereo or mono amplifier, signal is fed into the non-inverting input of the differential amplifier. The signal is then fed to the second gain stage pre-driver and then to the positive output driver. The inverting input of the differential amplifier receives input through both a dual diode network and the overall feedback of the amplifier circuit. This input is fed into the current mirror, inverted, fed into the second

gain stage pre-driver, and then into the negative output driver. The right channel operates in the same manner, but the input to the differential amplifier is reversed with the signal applied to the inverting transistor. The driver stage level-shifts the signals, from the low voltage operation of the input circuit, to the high voltage operation of the output stage. Drive current for the output MOSFETS is supplied by the driver transistors. In the case of multiple output MOSFETS, then the use of current amplifiers and buffers is necessary for both positive and negative sets. Class AB bias current is established by a bias transistor and potentiometer placed in parallel with the pre-driver transistors. When the pre-drivers are activated, the current draw between them is at a constant. Voltage across the drive stage is varied with the altering of the bias potentiometer, thus varying the current flow to the MOSFETS. With the pot in the full counterclockwise (off) position, the voltage level to the bias transistor is at a maximum, causing the transistor to turn on hard. This drops all of the current across the emitter resistor (10ohms), resulting in very low bias voltage to the outputs. In the other extreme, the potentiometer in the full clockwise position (full on), the bias transistor will no turn on, nor conduct, and thus all current is dropped across the resistor network between the pre-driver stage, resulting in a maximum bias voltage. This runs the amplifier into full class A operation causing possible damage to the output devices. Refer to figure 8.

figure 8. Driver stages

With the MOSFETS configured with a source-on-rail connection, the input signal, routed from the driver transistors, is amplified significantly to an output load. Feedback is fed back to the input via the differential pair.

Protection Circuitry
Our NOMAD (Non-multiplying Advance Decision) circuit ensures maximum power delivery, within the output MOSFET ratings for power, voltage, and current. The device current and voltage are electrically measured and the drive voltage is clamped to limit the power demand when the operation exceeds the device dissipation limits. NOMAD is comprised of separate, symmetrical sections, which monitor operating conditions of the positive and negative output devices.


In the figure below, the NOMAD circuit is implemented into the schematic. The voltage drop across R134 is proportional to, and represents, the positive output current. The voltage drop across R128 is a shaped function of the negative output device voltage. The emitter to collector voltage of Q108 is constant and supplies the circuit reference voltage. When the sum of these voltages exceeds 0.6 volts, Q106 is turned on. If the condition persists long enough to charge C115, Q107 is activated and clamps the output gate voltage. With the voltage the same across the gate and the source of the output transistor, current no longer flows, thus shutting off the device until the condition is removed.

figure 10. NOMAD protection circuit

Signal from the front end is introduced into the differential pair amplifier. The current source switches the differential pair on after receiving a Switching Delay from the turnon circuit. The non-inverted signal is sent through the positive pre-driver to the positive driver as the inverted signal is sent through the current mirror and then through the negative half of the circuit. With bias applied to the MOSFETS, the signal is routed to them via the driver transistors. The signal is then amplified and placed across a load speaker as audio. If, for any reason, a short or overload at the speaker terminals exist, the NOMAD protection circuit activates until the condition is reversed or removed.

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