Analog Top Talks in November 2019 on EDABoard.com

Peer-to-peer, engineer-to-engineer questions and answers from the EDABoard.com engineering community around analog ICs and analog design. Click the “Read more” link and follow the entire conversation and maybe add your two cents by logging in to EDAboard.com.

NPN having its VBE protected against reverse voltage – The attached circuits provide an 8.3V rail which supplies approx. 10mA. The Left-hand version shows a diode D1, which is supposed to protect the NPN. Our contractor tells us that this is essential. However, though I don’t like over-reverse-voltage NPN Vbe junctions, I find this diode D1 to be unnecessary in this case. Do you agree? Read more

$freq while using AC analysis to access lookup table – I have saved in a table the response of a filter. The table contains two columns; the first contains the frequency and the second the corresponding magnitude. I am building a model that will instantly “read” the value of the frequency during an AC analysis, and using that value will find the corresponding magnitude through the table. My thought was using something like $table_model( $freq, “table_link.dat”) and assigning that value to the output, but it doesn’t seem to work. It seems like the only value of frequency is 0Hz. Any ideas? Read more

2:1 multiplexer with open-drain outputs IC – We want to connect a GPIO (3.3V) to 2 devices (selected by another GPIO, one device is 3.3V and other device works with 1.8V. I see 2:1 mux with open-drain outputs so, that I can connect each output to any voltage (1.8V/3.3V) tolerant device. Any other alternate way to fulfill this? Read more

2SC1971 amplifier problem – I have just started getting into RF electronics, and I really wanted to try and build an amplifier for the FM band, since I have a small 250mW PLL FM transmitter.
I found a PCB layout on the internet that looked to me, pretty simple and easy to put together. I gathered the parts and threw it all together, and I think it turned out OK. The problem is that as soon as I turn the thing on, the power supply maxes out at 1.5A and the transistor gets pretty hot. I have checked for shorts multiple times and it’s driving me crazy since there are none. Read more

Reference for VCO and PLL – I’m working with a system that requires a 23.8 – 26.8 GHz VCO (HMC739LP4). During lab testing, to maintain stability I setup a phased locked loop using the divide by 16 output on the VCO, an HP frequency synthesizer as the reference, and a mini-circuits double-balanced mixer as the phase detector. This setup works great, but when I move towards the final product design, I will have to create my own stable reference ~1.45 – 1.6 GHz and move away from using a bench-top synthesizer. What are some of the common methods to create such a stable “tune-able” reference? Read more

Sampling rate vs rate of change of input for control system – I am referring to http://support.ctc-control.com/custo…rTheLayman.pdf to understand type ii servo loop. Theory provides zero velocity lag for changing velocity inputs. To implement in hardware, I wanted to know if sampling rate is to make higher than the input rate of change? Read more

Optimize a common source amplifier  – I need to optimize a Common Source Amplifier circuit as shown, with one single bias point, Vindc, maintain > gain of 15 dB @ saturation region. I’m allowed to manipulate the W/L / R_D / Vindc. While keeping the V_DD constant @ 1.2V. The circuit must work in -40, 27, 70 and 125’C at the corners ‘tt’, ‘ss’, ‘ff’, ‘fnsp’ and ‘snfp’. The problem arise caused by my corner and temperature settings, the Vout vs Vin generated is offset, and I couldn’t determine the best Vindc point. Read more

Charge pump with unity gain amplifier – I’m trying to understand the benefits of a charge pump with unity gain amplifier compared to the normal charge pump that is implemented using two current mirrors. Read more

Ground plane in audio amplifier – Can a ground plane, create problems in power audio amplifiers? For example, if touching the input with the tip of whatever, the amplifier oscillates, can it be the fault of the ground plane? Read more

Inverter output V-high voltage – I have designed a circuit with a 1.2V dc power supply. Part of the circuit is a clock generator and I output a few clock signals out to test. In order to drive more load, I adopted 4 series-connected inverter buffers before the output pad. When I have my IC back, I glued a piece to a PCB and wire-bonded to the 50 Ohms SMA to 50 Ohms oscilloscope. The phase relationship of the output is correct but the amplitude is wrong. Read more