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I t is a flexible device; t he frequency response can be shaped with some external components and there are plenty of examples of clever circuits that people have come up with over the years. Due to its low quiescent current drain and power consumption, it is suitable for portable battery-powered guitar mini amplifiers. Some of the best known are:. The voltage gain can be adjusted from 20 to 26 to 46 dB with a wide supply voltage range: 4VV or 5VV depending on the model.
The inputs are ground referenced while the output automatically biases to one-half the supply voltage. It has a low quiescent current drain: 4mA 24 mW when operating from a 6 volt supply and "low" harmonic distortion: up to 0. The internal circuit is based on a classic audio power amplifier configuration typically referred as Lin Topology. Although old, it remains nearly unbeatable and almost all solid-state power amplifiers follow it. Combining two complementary transistors the emitter followers can be connected in a push-pull configuration where each transistor amplifies the current of its corresponding half wave.
Such topology is known as Class-B amplifier, very efficient but subject to crossover distortion. A further improvement on the VAS is to upgrade it to Class-AB using a pair of diodes, reducing the efficiency but vastly improving the crossover distortion.
The first block is a PNP Emitter Follower amplifier Q 1 , Q 3 , it sets the input impedance and defines the DC operation points, raising the input voltages off the ground so the circuit will accept negative input signal down to Both 50k input resistors R 1 , R 3 create the path to ground of the base current, the input needs to be coupled so not to disturb the internal biasing, hence the input impedance is dominated by these resistors and set to 50K.
Voltage Gain Analysis:. The differential amplifier Long Tailed Pair Q 2 , Q 4 gain is adjusted by two gain-setting resistors 1. External pins 1 and 8 provide access to adjust the gain from 20 min to max.
The voltage Gain can be calculated under quiescent conditions no input signal applied as follows:. Due to the current mirror, the current intensity through R8 is equal to 2I, neglecting the current i7 through the two 15K resistors R6, R7 , which are large impedances compared to other parts of the circuit, thereby:.
This formula can also be rewritten in a more generic way as:. Where Z and Z are the impedances between the respective pins. Crossover Compensation:. The diodes D 1 and D 2 are used to compensate the crossover distortion.
In order to counteract the transistor minimum limit to conduct Vbe , they are biased so their idling voltage never drops below the forward voltage Vbe.
Using a diode proved to be one of the best solutions: It offers a voltage drop which is temperature dependent and by matching the thermal coefficiency with the transistor the bias current can be kept quite stable. If an accurate thermal tracking is required the diodes are mounted to the same heatsink as power transistors. Since one diode usually is not enough, amplifiers often use several diode junctions, two in this case. Negative feedback is applied from the output to the emitter Q 4 via resistor R 8.
This DC feedback acts to stabilize the output DC bias voltage to one-half the supply voltage. Qualitatively, the dc feedback functions as follows: If for some reason Vo increases, a corresponding current increment will flow through R8 and into the emitter of Q 4. Thus the collector current of Q 4 increases, resulting in a positive increment in the voltage at the base of Q 7. This, causes the collector current of Q 7 to increase, thus bringing down the voltage at the base of Q 7 and hence Vo.
And now there are 2 approaches to get the same conclusion:. Approach The current mirror Q 5 , Q 6 balances the LTP, equalizing the current through both transistors Q 2 , Q 4 and improving the linearity of the input stage. Therefore, the currents on both tails are equal: both DC and AC components.
Supplementary external components can be used to tailor the response to specific applications. The LM application data mentions a bass boost by connecting an RC network between pins 1 and 5 paralleling the internal 15k resistor :.
The amplifier is stable only for closed-loop gains greater than 9, so if the external resistor R is too small, the circuit could oscillate. Thereby, the minimum R can be calculated easily:.
Actually, this modification did not provide an active boost, just a roll-off at frequencies below the selected frequency, that is it a low pass filter. Unfortunately, the bass boost will be gain dependent. So f c can be calculated:.
So, assuming that the Z internal resistance is 15K, the value of f c for the most common RC values can be calculated as follows:. JRC Datasheet. LM Datasheet from National Semiconductor. Thanks for reading, all feedback is appreciated. Some of the best known are: Smokey Amp , the smallest and least expensive. Uses only 2 components and it is able to fit in a cigarette package.
Ruby Amp , adds an input buffer to the Little Gem and also improves other sections of the circuit. Table of Contents: 1 Electrical Characteristics. Electrical Characteristics. Chip Name Min VolT. Max Volt. Out Power Typ. LM Internal Circuit Analysis. I ts major tasks are: Define the DC operating points. Set a high input impedance.
To subtract the feedback signal from the input path in order to reduce distortion. Its job is to amplify the low-level signal generated by the input signal to a suitable level. A basic VAS circuit is a common emitter amplifier. The function of the output stage is to provide enough current gain so that voltage potential provided by VAS can exist over a low impedance load.
The simplest current amplifier is an emitter follower. Feedback can be either local, global or a mixture of both.
The voltage Gain can be calculated under quiescent conditions no input signal applied as follows: Note: The voltage across R4 and R5 Vdiff is simply the differential input voltage Vin because the base-emitter voltage drops in the PNP transistors Q1, Q2, Q3, and Q4 are the same in each side of the LTP. This current is labeled "I". And now there are 2 approaches to get the same conclusion: Approach 1: The current mirror Q 5 , Q 6 balances the LTP, equalizing the current through both transistors Q 2 , Q 4 and improving the linearity of the input stage.
LM Frequency Response. The LM application data mentions a bass boost by connecting an RC network between pins 1 and 5 paralleling the internal 15k resistor : The amplifier is stable only for closed-loop gains greater than 9, so if the external resistor R is too small, the circuit could oscillate.
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